Embryo and Vegetative Organs

should not be cut too thin. Mount in glycerine, and examine with a low power. ALISMA. EMBRYO. 135 One of the sections will probably be found to include 1. The Embryo-sac, in which are contained 2. The Embryo, with a very short suspensor, the basal cell of which, is greatly enlarged. 3. The Endosperm, a tissue which lines the embryo- sac, and varies in appearance according to the stage of development of the ovary. II. GYMNOSPERMS. VEGETATIVE ORGANS. EXTERNAL CHARACTERS. Take a branch of Pinus Sylvestris, cut in autumn, and which includes at least four years' growth. N.B. The limits of each year's growth may be recognised externally as those points where (false) whorls of strong lateral axes are developed ; and the portion of stem lying between two such whorls may be considered roughly as representing one year's growth. I. Consider first the growth of the year in which the branch was cut, i.e. the part above the youngest whorl of lateral axes. At its apex is a large Bud, surrounded by a variable number of smaller Lateral buds. From a bud, which has been treated with alcohol to remove the resin which covers it, detach some of the brown Scale-leaves, which cover it externally. Note 1. The succulent base of these scales. 2. Buds in their axils. Compare these winter buds with some of the same which have been cut in late spring. The brown scale-leaves will be found to have fallen off, leaving their succulent bases still persistent ; in PINUS. VEGETATIVE ORGANS. 137 the axils of these will be seen the axillary buds above noted. The main axis of the bud has become elongated by extension of the tissues. In studying the growth of the current year, bear in mind that it has been derived from a bud, which had a similar structure to that which is now seated at its apex. Examine the stem of the current year externally, and note 1. The thick Main axis, more or less succulent in appearance; its surface is marked by longitudinal grooves. 2. The brown tooth-like bases of the scale-leaves of the bud, best seen at the lower part of the intern ode. 3. In the axils of these, especially at the upper part of the internodes, are Axillary buds of two kinds. a. Buds with limited growth, each bearing two acicular foliage leaves, surrounded at the base with numerous scale-leaves. These limited foliage shoots occur in the axils of the scales throughout the greater part of the current year's growth. b. Buds with unlimited growth, which are seated close to the apex of the shoot of the current year. They are few in number ; their structure has already been observed ; each may develop into an unlimited axis. It may here be observed that both a and b have a similar origin, both being axillary buds in the axils of the leaves of the main axis of the current year. The apparent difference depends upon the fact that the buds b are more strongly developed than a. 138 PRACTICAL BOTANY. There is great variety in the character of the leaves in the Conifercc. In some cases only foliage leaves are developed (Araucaria, Juniperus, Thuja). In one case only scale leaves are formed (Phyllocladus), while here in Pinus we have both scaleand foliage-leaves, the former alone being borne on the stronger axes with unlimited growth, while the latter appear only on the foliage shoots with limited growth. Specimens of different members of the group should be examined and compared. II. Passing to the increment of growth of former years, i.e. to the lower and older parts of the branch, in the external appearance and arrangement of parts they resemble that of the current year. The main axis increases in thickness, and is more obviously ligneous, while the limited foliage shoots drop off, leaving scars which mark their former position. HISTOLOGY. THE STEM. It is best to work with material which has been treated for some time with spirit ; by this means the resin, which would otherwise clog the razor, is removed. I. Cut transverse sections of the Axis of a bud, and treat with dilute potash for a few minutes : mount in glycerine. Meanwhile other sections may be mounted in Schulze's solution: examine with medium or low power, and observe at the centre of the section 1. The Pith, composed of cells, with intercellular spaces, and thick cellulose walls (blue with Schulze's solution) ; surrounding this a series of groups of smaller constituents : these are 2. The primary Vascular bundles. Note that they are PINUS. STEM. 139 a. Separated from one another laterally by bands of parenchyma. b. Their form is approximately wedge-shaped. c. That the tissues of which they are composed may be distinguished as A i. Xylem portion, nearer the centre of the stem, the components of which have thick, dark-looking, lignified walls (yellow with Schulze's solution). These first formed xylem elements, since they differ from those formed later, are distinguished as Protoxylem. A ii. Phloem portion, nearer the periphery, with bright-looking cellulose walls (blue with Schulze's solution). The more minute study of these tissues must be deferred for the present. Outside the ring of vascular bundles is 3. The Cortical tissue, a mass of cells similar in structure to the pith. In this occur large intercellular spaces, which are Resin-passages. Since the periphery of the section of the axis of the bud is complicated by great irregularity of outline, the study of the outer tissues will be better carried out in the older stem. II. Cut transverse sections of the stem of the current year. Mount some in glycerine, others in Schulze's solution. The sections have a wavy outline, the in- dentations corresponding to the grooves above observed externally. Starting from the periphery of the section, note the following tissues : 1. Epidermis, a single layer of cells, following the wavy outline of the section : the walls, especially the outer, much thickened : externally a Cuticle. 2. Cortical tissue, consisting of cells with rather 140 PRACTICAL BOTANY. thick cellulose walls (blue with Schulze's solution), and protoplasmic contents with chlorophyll. Many cells have recently divided (this is necessary to keep pace with the growth in thickness of the vascular cylinder). Large intercellular spaces (Resin-passages) occur here and there, and are lined with small-celled epithelium. It must be remembered that in the present case the resin itself has been dissolved out by alcohol. Sections should, therefore, be made from fresh material in order to see the secretion in situ. It appears amorphous and transparent ; it is soluble in alcohol, leaving a slight residue. N.B. All resins are not so easily soluble. The secretion stains deeply with tincture of alkanet. Near the periphery of the cortex will be found a- layer of Cork and a Cork-cambium (cf. stem of Elm p. 70), derived from cells of the cortex by their division by tangential walls. The cells of the cork have no cell-contents ; their walls are coloured yellowish brown with Schulze's solution. Treat a section with strong sulphuric acid. The walls of the cork retain their sharp contour. At the bases of the indentations of the margin of the section, and immediately below the epidermis note groups of Sclerenchyma, having thick lignified walls (yellow with Schulze's solution). 3. The Vascular system : here a complete ring (cf. the bud) : distinguish as before (a) the external Phloem, (6) the internal Xylem, (c) the misty layer of Cambium. N.B. The vascular bundles were seen to be separated in the bud by intervening parenchyma. Here the ring PINUS. STEM. 141 has been completed by the formation of an Interfasci- cular cambium in the parenchyma between the original bundles. Observe that the internal limit of the vascular ring is sinuous. The convexities mark the position of the primary bundles ; at the apex of these will be found the Protoxylem. 4. The Pith, consisting of parenchyma, having the same characters as in the bud. No resin-passages. Put on a high power, and examine the Cambium. Note i. That the cells are arranged with great regularity in radial rows. ii. That their walls are thinner than those of the surrounding tissues, and are composed of .cellulose (blue with Schulze's solution). iii. That the tangential walls are thinner than the radial. iv. That the cells have copious protoplasm, in which a Nucleus may often be recognised. These facts point to a repeated division of cells by tangential walls. Draw carefully, and compare several of the radial series of cells of the cambium. They will be found to coincide with Sanio's law of cambial division, which was first concluded from obser- vations on Pinus sylvestris. Observe, here and there, radial rows of which the cells are more elongated in a radial direction than the rest. These may be traced outwards towards the cortex and inwards towards the pith. They are the Medullary rays. Some of them may be traced the whole way to the cortex and to the pith (primary medullary rays) f 142 PRACTICAL BOTANY. others only part of that distance (secondary medullary rays). Note that the cells of the medullary rays at the cambium zone are less elongated radially than in the xylem or phloem ; the cambium being the formative point of these tissues. The mature cells of the ray usually have cellulose Avails (blue with Schulze's solution), and granular protoplasmic contents with nucleus. In fact the cells of the medullary rays usually retain their cell-nature. Follow the radial rows of cambium cells outwards, and note the gradual transition to the permanent tissues of the Secondary phloem, the constituents of which are also arranged in radial rows, and have cellulose walls (blue with Schulze's solution). The ring of secondary phloem is cut up into rectangular areas by the Medullary rays, which are easily recognised as above directed. Observe that the tissues filling these areas are of three sorts. i. Elements with cellulose walls, and no very distinct contents ; they are radially compressed. These are the Sieve-tubes, which compose the greater part of the phloem. The walls are differentiated into layers, and have bright globules attached to them (yellow with Schulze's solution). ii. Here and there the radial rows of sieve-tubes are broken by single large cells of the Bast-parenchyma, which resemble in their characters those of the medullary rays. iii. Towards the periphery of the phloem are elements similar in form to the sieve-tubes, whose cell contents are brown, and contain crystals. PINUS. STEM. 143 Note on passing to the periphery of the phloem an increasing irregularity of form of the tissues due to distortion, caused by pressure from without by the cortical tissue upon the vascular system, as it increases in bulk by secondary thickening. Sclerenchymatous elements are absent from the phloem of the stem of P. sylvestris. They are, however, found in the phloem of many of the Coniferce, e.g., Juniperus, in which the different tissues are arranged with great regularity. Follow the radial rows of cambium cells inwards, i.e. towards the centre of the stem. Note the transition from thin-walled cambium to the thick-walled tissue of the Xylem. If the stem was cut in winter the transition will appear sudden, if cut in summer it will appear gradual. The tissue-elements retain the same arrangements in radial rows, as the cells of the cambium. Observe that the xylem ring is cut by the medullary rays into wedge-shaped areas. The chief tissue-elements filling these areas are the Tracheides, which present the following characters : i. They have approximately the same shape as the cells of the cambium from which they are derived. ii. Their walls are thick and lignified (yellow with Schulze's solution), and are differentiated into layers distinguished optically, and by staining. iii. They have no cell-contents. iv. On their radial walls (and rarely on the tan- gential walls) are found irregularities of structure called Bordered pits, which are best seen in the xylem formed at the early part of the year. Note the pit-membrane, 144 PRACTICAL BOTANY. which is always present, traversing the pit-cavity in all cleanly cut sections; the pits are therefore not per- forated. Observe near the centre, and bordering on the pith, the Protoxylem arranged as above observed in the younger stem. No bordered pits occur in the walls of the protoxylem. Note the occurrence of resin-passages in the secondary xylem, lined as before by thin-walled epithe- lium, which may be regarded as xylem-parenchyma. III. Cut transverse sections of a three years old stem so as to include the whole width of the vascular ring. It is not necessary, however, to have a complete transverse section of the whole stem. Mount in glycerine. Comparing this with what has already been observed in the stem of the current year, note the following differences : 1. The cortical tissue bears evident traces of tangential extension. This is necessary to keep pace with the increase in bulk of the vascular system. 2. The phloem is thicker, and the constituents of the outer part of it are much distorted and displaced. 3. The xylem has increased, in thickness more than any other tissue, so that it is now the chief constituent of the stem. It may be distinguished as being composed of three bands (annual rings), in each of which the more central tracheides have large cavity and thinner walls (wood developed in spring) ; passing outwards there may be seen a gradual reduction of the cavity, and increase in thickness of the walls till a certain limit is reached (autumn wood). Outside the latter is a sudden PINUS. STEM. 145 transition to the spring wood. At this point is the limit of each year's growth. IY. Cut radial longitudinal sections of a three years' old stem. Mount some in glycerine, others in Schulze's solution. The sections should be accurately radial and longitudinal, otherwise the difficulty of study of the tissues is greatly increased. Beginning at the centre of the stem and passing outwards observe successively : 1. The Pith, consisting of two sorts of elements, both of which are of parenchymatous form. a. Cells with cellulose walls (blue with Schulze's solution) pitted, with protoplasm and nucleus. 1. Elements of similar form with pitted lignified walls, and no cell-contents. From the arrangement of these it may be concluded that they had a common origin. 2. The Xylem consisting of a. Tracheides with lignified walls, and no cell- contents. Starting from those nearest the pith (i.e. from the protoxylem), and passing outwards, the following forms may be observed, and distinguished mainly by the markings due to unequal thickening of the walls. i.--Tracheides with narrow cavity, and more or less regular annular or spiral marking. ii. Elements wider than these, and with bordered pits scattered between the spirals. iii. Normal Tracheides with bordered pits only. These form by far the greater bulk of the secondary xylem, and must be carefully studied. Their form is prosenchymatous. The greater part of the L 146 PRACTICAL BOTANY. cell-walls is of uniform thickness. On these por- tions of the wall observe with the highest power two intersecting systems of lines of striation. In single longitudinal rows are found the Bordered pits ; each of these appears as two concentric rings, of which the smaller is more strongly marked, and corresponds to the opening of the pit into the cell-cavity. It must be remembered that we are now observing the radial walls in surface view. Compare the bordered pit as seen here with its appearance when seen in transverse section. Note the annual rings recognised here, as in the transverse sections, by difference in width of cavity, and thickness of walls of the tracheides of the .xylem. 5. Here and there the continuity of the mass of tracheides is broken by a longitudinal resin-passage, surrounded by parenchymatous cells (xylem-parenchyma), which have cellulose walls and retain their cell-contents. The whole mass of xylem is traversed radially by plates of parenchyma (Medullary rays). Note that they extend only a short way longitudinally, but a long way radially ; also that they are composed of cells arranged like bricks in a wall, among which may be be distinguished a. Cells with cellulose walls, and protoplasmic contents. The walls of the tracheides which abut on these are unusually wide. b. Elements, with no protoplasm, and with lignified walls marked with bordered pits. Both tissue-forms may often be found in the same ray, though rays will often bo seen consisting of (b) alone. PINUS. STEM. 147 3. The Cambium-layer consisting of elongated thin- walled cells, the ends of which are difficult to observe (cf. tangential sections). They have copious protoplasm, and an elongated nucleus. Note that the medullary rays are continuous through the cambium, and observe the differentiation from the uniform cambium of the ray to the forms (a) and (ft). In the sections through the cambium of a stem cut in summer, the development of the bordered pits on the walls of the tracheides may be studied. The sections should be treated with Schulze's solution for a long time. 4. The Phloem tissues, which are best studied in sections, which have been treated for some hours with Schulze's solution, consist of a. Sieve-tubes, elongated structures with cellulose walls, those which are radial being marked by numerous circular sieve-plates, here seen in surface view. These sometimes stain a sherry brown with Schulze's solution. The ends of the tubes are difficult to observe (cf. tangential sections). Their protoplasmic contents are transparent and sparing. I. Bast-parenchyma, cells arranged in longitudinal rows, with cellulose walls, and copious protoplasm. c. Occasional elements (prosenchymatous or parenchymatous) with brown cell-contents, in which crystals are embedded. These are found towards the periphery of the phloem. Medullary rays will be seen with a similar arrangement to that seen in the xylem. Their cells, which resemble those of the phloem parenchyma in character, are all alike. 5. Externally to the phloem is the cortical parenL2 148 PRACTICAL BOTANY. chyma, which requires no further notice here. Outside this is cork (and sclerenchyma at certain points), and at the periphery of the section ~ 6. The Epidermis. V. Cut tangential sections of a three or four years' old branch, and bear in mind that as a rule the central part of the sections is the most accurately tangential, i.e. that the plane of section is there most accurately perpendicular to the radius of the stem. Mount as before. A, In sections which pass through the peripheral part of the xylem observe i. The Tracheides of prosenchymatous form. No bordered pits (or very few) are seen in surface view, but they may be seen in large numbers in the radial walls (here cut longitudinally) presenting a similar appearance to that seen in transverse sections. ii. Medullary rays, which resemble a section of a biconvex lens. Note that each ray extends only a short distance in a longitudinal direction : in some cases rays consist of only a single radial series of cells, of which only one lenticular cell appears in this section. Occasionally a resin-passage is included in a ray. iii. Longitudinal resin-passages (cf. radial sections). B. In sections passing through the cambium will be seen i. The Cambium-cells, resembling the tracheides in form (prosenchymatous) ; cell-walls thin ; granular, with elongated nucleus. ii. Cambium of medullary rays, protoplasm similar in shape to the cells of the rays : thin-walled, with granular protoplasm and nucleus. PINUS. LEAF. 149 If these sections be treated with dilute potash, and mounted in glycerine, their structure may be more easily made out. C. In sections passing through the phloem will be seen i. The Medullary rays as before, but their form is more convex: all the tissues between the medullary rays are derived from cambium-cells of the form above observed. These are ii. Sieve-tubes, which retain the form of the cambium-cells : the cellulose walls seen in surface view are smooth : those cut longitudinally appear of wavy outline (sieves). The structure of the latter is well seen after treatment with Schulze's solution for twenty-four hours. Contents transparent protoplasm. iii. Bast-parenchyma, derived from cambium-cells by their division by transverse walls. iv. Some few cells, especially towards the periphery, containing crystals which give the reactions of calcium oxalate. THE LEAF, Cut transverse sections of a foliage leaf of Pinus syhestris, taken from a stem of the current year. It may be found convenient to embed in paraffin, or to hold the leaf between pieces of pith, or carrot. Mount as before, and examine with a low power. Note the form of the section ; the flat side is the upper, the convex side the lower. Observe successively the following tissues : A A. single layer of Epidermal cells with very thick walls. B. A narrow band of thick-walled Hypoderma. 150 PEACTICAL BOTANY. A C. broad band of chlorophyll-containing Meso- phyll, with resin-passages. A D. Bundle-sheath, consisting of oval cells. A E. broad band of tissue without chlorophyll, which surrounds F. Two central Vascular bundles. Study these several tissues under a high power. A. The Epidermal cells have their thick walls differentiated into three layers. These may be recognised without staining, or better after treatment with Schulze's solution, as A i. thin external Cuticle, not very deeply stained. It extends as wedge-like processes between the cells. ii. The Cuticularised layers, forming a thick band, which stains a deep brown. Immediately surrounding the cell- cavity is A iii. broad pitted band, not deeply stained. This differentiation may be brought into greater prominence by treating (a) with strong sulphuric acid, or (6) by staining slightly with fuchsine. (c) If sections are boiled for ten minutes or more in strong solution of potash, i. will be dissolved while ii. and iii. remain. Note the larger cells at the angles of the section, with thicker walls. Here and there depressions of the external surface may be observed. These indicate the position of the Stomata. Observe the two guard cells, which are seated some distance below the surface of the leaf. B. The Hypoderma (sclerenchymatous) varies in thickness from a single layer of cells to several layers. It is thickest at the corners of the section cells thick- ; PINUS. ROOT. 151 walled, lignified. stomata. Note that it is absent below the C. The Mesophyll consists of thin-walled, chlorophyll-containing parenchyma. The cellulose walls (blue with Schulze's solution) show a peculiar infolding. Resin-passages occur in it. Their cavity is lined with thin-walled epithelium, which is immediately surrounded by a layer of thick-walled sclerenchyma. D. The Bundle-sheath, walls stained brown with Schulze's solution. E. The tissue within this consists of two elements : i. Parenchymatous cells, with thin cellulose walls (blue with Schulze's solution), and protoplasmic con- tents. ii. Elements having lignified walls, with bordered pits, and no cell-contents (tracheides, transfusion-tissue. [Mohl.]). F. The two central Vascular bundles, the constituents of which resemble those of the stem. Note that the xylem is directed towards the upper surface. Thick-walled sclerenchyma is scattered irregularly round the bundles. THE ROOT. I. Cut transverse sections of a young primary root of the seedling of Pinus (not necessarily P. sylvestris) ; treat with dilute potash, and mount in glycerine. Observe : A 1. thick band of Cortex, not covered externally by any true epidermal layer (cf. longitudinal sections of apex of root). A 2. Bundle-sheath within the cortex. This is a 152 PRACTICAL BOTANY. single layer of cells, having the characteristic marking on the radial walls. Within this lies 3. The Pericambium, a band three or four layers of cells thick. This immediately surrounds 4. The central Vascular cylinder, in which may be seen a. Y-shaped groups of Xylem elements, the fork of the Y directed outwards ; their number varies (3 6). Be- tween the limbs of the fork of each lies a resin-passage. I. Groups of Phloem elements, equal in number to the xylem groups, and alternating with them. N.B. These tissues of the phloem are not very easily recognised. c. Centrally is a mass of parenchyma, which also extends between the xylem and phloem masses, and separates them from one another. II. Cut other sections from an older part of the root, and treat as before. Observe that : 1. The cortex becomes disorganised and brown. 2. Divisions appear in the outermost cells of the pericambium, forming a layer of cork. 3. Lateral roots may occasionally be found, origi- nating in the pericambium, opposite the xylem. 4. The parenchyma lying centrally to the phloem groups has begun to divide as a Cambium-layer. III. Cut transverse sections of a thin lateral root (about -j- of an inch in diameter) of a full-grown tree of P. sylvestris ; mount some sections in glycerine, others in Schulze's solution. Observe successively, starting from the periphery of the section : 1. Withered remnants of the Cortex. This may, however, have been already completely thrown off. PINUS. ROOT. 153 2. The Pericambium, with its secondary products arranged thus : a. Externally a thin band of Cork, the cells of which are arranged in radial rows. b. The Cork-cambium, the cells dividing by tangen- tial walls. c. The remainder of the original pericambium in a quiescent state. 3. The Phloem, forming, according to the age of the root, a more or less complete ring. The constituents resemble those of the phloem of the stem, and are often distorted by external pressure. 4. The Cambium, as in the stem. 5. The Xylem, in which may be recognised, near the centre a. The primary xylem groups, arranged in the form of a Y, and each having, as before, a resin-passage in the fork. I. The masses of secondary xylem, more or less fan-shaped, and alternating in position with the groups of primary xylem. The number of the latter, and of the masses of secondary xylem, varies in the lateral root, four being the average number. The constituents of the secondary xylem resemble those of the stem in structure and arrangement. IV. Cut, and mount as before, transverse sections of a root about one-eighth of an inch in diameter. The arrangement of tissues will be as before, but the fan-shaped masses of secondary xylem will have joined laterally, so as to form a complete ring. Annual rings may also be seen in fact, the structure at the periphery of the root now closely resembles that of the stem. 154 PRACTICAL BOTANY. V. Cut median longitudinal sections of the apex of the root of Pinus. This may be easiest done by cutting longitudinal sections of the mature embryo in the seed. Treat with potash till they are transparent, and mount in glycerine. Observe : 1. The central Plerome cylinder, recognised as in the Sunflower and the Maize. It is Bounded off at the apex, and throughout is quite distinct from 2. The Periblem, which surrounds it. This is the formative tissue of the cortex. Outside this no true epidermis is to be found ; but at the apex is A 3. Root-cap, which is formed by the active division of the cells of the periblem at the apex of the root. Compare this arrangement of the apical meristem with those types seen in the roots of Angiosperms. REPRODUCTIVE ORGANS. We have seen at the apex of the ordinary vegetative branch in spring, an apical bud surrounded by a number of lateral buds, all of which normally develop into vegetative axes of the type above described. The reproductive organs of Pinus are produced on buds corresponding in position to these : they are easily recognised, even at an early stage of development, with the naked eye. The following observations should be made upon museum specimens, otherwise they oould only be made at intervals, according to the .period of development of the organs in question. I. Male inflorescence. A. Note that the inflores- cence while young, appears as a bud covered with brown PINUS. REPRODUCTIVE ORGANS. 155 scale-leaves, in the axils of which are lateral axes easily seen on removing the scales. Of these lateral axes (a.) Those nearest the apex of the bud develop as lateral foliage-shoots (cf. ordinary vegetative axis). (b.) Below these, a number bear, in place of the two foliage leaves, numerous Staminal leaves (these axes are Flowers). Comparing the male inflorescence with the ordinary vegetative axis, the main difference lies in the mode of development of the lateral axes. In autumn the male inflorescences of the preceding summer can only be distinguished from the purely vegetative axis, by the absence of the lateral foliage-shoots from the lower parts of them. B. Separate a single male flower, and cut it longitudinally in a median plane : it will be found to consist of 1. An Axis, which bears. 2. A number of Staminal leaves. Detach some of these staminal leaves with a needle : each consists of A (<z.) short stalk, or Filament, which bears at its apex (6.) An expanded Anther, with two swellings (Pol- len-sacs, or Microsporangia) on the lower surface. 0. Cut longitudinal sections of the male flower in which the pollen is not yet ripe, and mount in glycerine : examine with low power. Note the arrangement of the parts as above described. In the pollen-sacs note the Pollen-grains in situ (Microspores). The pollen is ripe about the middle of June, and material ghoul d be collected and preserved in alcohol at such time as to 156 PRACTICAL BOTANY. illustrate various stages of development (i.e., at short intervals during May and June). By cutting sections from such material, and treating as above directed, the history of development of the pollen may be made out. D. Mount ripe pollen-grains (i.e. such as may be collected by shaking a male branch in June) in dilute glycerine, having previously wetted them with alcohol. Observe 1. The two large lateral Wings, usually filled with air, which facilitate the carriage of the pollen by the wind. These are extensions of the outer coat (extine). 2. The central body of the pollen-grain consisting of A (a.) large cell, which constitutes the greater part of the grain, and from which the pollen-tube springs. A (b.) series of one or more smaller vegetative cells, affixed laterally to the wall of the pollen-grain at a point between the wings. These take no direct part in the formation of the pollen-tube. II. Female branches or Cones. Observe on a Scotch Fir, towards the end of June, that there are cones to be found in three different stages of development, the position of which is constant. (a.) Small green cones occurring (one or more) close to the apex of the shoot of the current year. Note that the basal part, or stalk, bears brown membranous scales, while the upper part is globular, and is marked out into numerous square areas, which are the apices of so many Ovuliferous scales. Comparing a shoot, which bears such young cones, with an ordinary vegetative shoot, it will be seen that the cones correspond in position to the lateral buds, of which they are the morpho- logical equivalent. PINUS. REPRODUCTIVE ORGANS. .157 (&.) Larger green succulent cones, which occur laterally at the apical part of the shoot of the previous year: the arrangement of parts on these corresponds to that on (a). (c.) Cones larger than (&), brown and with lignified tissues : on these the scales are usually more or less separated from one another so as to disclose the seeds, two of which are borne at the base of each of the ovuliferous scales. These ripe cones are seated laterally near the apex of the two-years-old shoot, A. Cut median longitudinal sections of a cone corre- sponding to stage (a). It should previously have been hardened with alcohol for some days : mount in glycerine, and examine with a low power. Observe 1. The central axis, not differing essentially from the young vegetative axis : on this are borne scales of two orders easily distinguished by their size. 2. The smaller of these are the leaves borne by the axis of the cone, and the morphological equivalents of the brown scale-leaves which cover the winter buds. In the axil of each of these is borne one of 3. The larger or Ovuliferous scales, which are longer and more bulky than (H) : they alone can be seen externally. On the upper surface of each of these, close to the axis, are borne 4. Two Ovules, or Macrosporangia, which are anatropous, so that the micropyle is directed towards the base of the scale : if cut in a median plane, each ovule will be seen to consist of i. One Integument, several layers of cells thick, with a widely open Micropyle facing the axis this ; is 158 PKACTICAL BOTANY. ii. The Nucellus, a mass of parenchyma, near the centre of which is iii. The Embryo-sac or Macrospore, a cell much larger than those of the surrounding tissue and lying some distance below the apex of the nucellus. Pollen-grains may often be found seated on the apex of the nucellus, one or more of these may throw out Pollen-tubes, which penetrate into its tissue. Dissect off one whole Ovuliferous scale, and observe on its upper surface, close to the base, two Ovules, which are anatropous. Note also the relative positions of the two sets of scales. B. Take cones of the stage above described as (b). The materials should be collected about the middle of June, and must be hardened in alcohol. Strip off the ovulifero^is scales of such cones. The ovules will remain adherent to the base of each. Cut longitudinal sections of the scales so as to pass through the median planes of the ovules ; mount in pure glyce- rine, and examine with a low power. Observe 1. The structure of the Ovuliferous scale, which is traversed by vascular bundles, and resin-passages. 2. The Ovule, which is united with the scale, and consists, as in the younger stage, of a. An external Integument. Note the wide micropyle ; I. The Nucellus; c. The Embryo-sac filled with Endosperm. All the parts of the ovule are larger than in the younger stage, but retain the same relative positions. Note carefully that Pollen-grains (one or more) are usually PINUS. REPRODUCTIVE ORGANS. 159 to be found lying on the apex of the nucellus ; and that from the larger cell of each of them arises a Pollen-tube, which traverses the tissue of the nucellus, as far as the apex of the endosperm, where it widens out into a large sac. Observe near the apex of the endosperm, and embedded in it, one or more large vacuolated protoplasmic bodies ; these are the Egg-cells, or Oospheres. From the apex of each may be traced a narrow neck or channel, inclosed by smaller cells than those of the surrounding endosperm. The neck and central cell together form the corpusculum (that is the archegonium). C. Remove ovules from cones of the second year taken and preserved in alcohol about August 1. Dissect off from them the now hardened Integument (Testa). Note within this the delicate remnant of the Nucellus, which covers the mass of Endosperm. Soak the latter in water, and dissect from it with needles the Embryos (numerous), which lie in the central cavity of the endosperm; treat them with potash, and mount in dilute glycerine. Examine with a low power, and observe 1. The Suspensors, coiled filaments consisting of numerous transparent thin-walled cells. At the ends of the suspensors are borne 2. The Embryos ; they are more or less elongated, almost cylindrical bodies ; in some cases (only one as a rule in each seed) they may have already formed a. An Apical cone, which terminates the free, an- terior end of the embryo ; this being surrounded by A b. whorl of Cotyledons of variable number. 160 PRACTICAL BOTANY. c. The apex of the Radicle, directed towards the suspensor (i.e. towards the micropyle of the ovule), and embedded in the tissue at the posterior end of the embryo. Note that there is no definite boundary between the suspensor and the embryo. Also that though polyembryony is the rule, i.e. a number of embryos are at first formed simultaneously, one of these supersedes the rest, and that one alone becomes differentiated as above described. By comparing sections of ovules of various ages (i.e., taken between the dates above named), cut and treated in the manner described for the cones taken in June [p. 158], the history of the early stages of development of the suspensors and embryos from the fertilised egg-cell may be traced. Ripe Seed. Examine the ripe seed of P. sylvestris, or other species e.g. P.pinea; and note the external hard and thick Testa; within this the Endosperm, which incloses the single Embryo. It has numerous Cotyledons and Radicle, the apex of the latter being directed towards the micropyle. Germination. Compare plants in different stages of germination, and observe the following points in the process : 1. The endosperm swells, and bursts the testa. 2. The radicle protrudes, and curves downwards. 3. The cotyledons elongate, and push out the stem, and their own basal portion* PIN US. GERMINATION. 161 4. The seed is usually carried upwards on the apex of the cotyledons, which, with the hypocotyledonary stem elongate greatly. 5. The plumule develops, forming numerous acicular leaves. N.B. The cotyledons turn green while still protected from the light, below the soil, and within the testa. PTERIDOPHYTA. A.-LYCOPODIN.E. I. SELAGINELLA MARTENSII. SPOROPHORE. I. In a well-grown plant note with the naked eye the following external characters : (1.) The Stem ascending, frequently branched, appa- rently in a dichotomous, but really in a monopodial manner (see below ); the branching occurs only in a single plane. (2.) The Leaves simple in form, with a ciliate margin, and arranged in alternating pairs; each pair consists of a dorsal and a ventral leaf, the whole series thus forming four orthostichies : note the two different sizes of leaves a. The larger ventral leaves, arranged in two orthostichies, without terminal awns. b. The smaller dorsal leaves, also arranged in two orthostichies, each leaf being terminated by a fine Awn. Each leaf has a single central nerve or Midrib. Turn back one of the leaves, and observe with a lens SELAGINELLA. SPOROPHORE. 163 the small scale-like Ligule. Note that the insertion of each leaf is oblique. (3.) The Rhizophores, long cyclindrical branched organs, which arise at the points of branching of the obliquely ascending stem, and grow vertically down- wards: note their frequent bifurcations. N.B. Two rhizophores are formed at each branching of the axis, one on the dorsal, and the other on the ventral side; of these only the latter is developed beyond the first rudimentary stage. Remove carefully a rhizophore, which has grown down so as to reach the soil, and wash it : observe (4.) The delicate bifurcating Roots, which rise at the point where the rhizophore touched the soil. Observe further that many of the branches of the stem may have a symmetrical arrangement of the leaves close to the apex : these are the branches or Cones, which bear the Sporangia : note that on these cones The leaves are all similar to one another and of (i.) small size. That (ii.) they are arranged in four symmetrical orthostichies. (iii.) That, on turning the leaves back, one Sporan- gium will be disclosed in each case. On comparing a number of sporangia, which have been exposed in this way, it may be seen that there are two sorts of them (a.) Macrosporangia, which are of a green or light-brown colour, and appear to be of rounded tetrahedral form. (&.) Microsporangia, which are more nearly spherical, and of a reddish-brown colour. Note in older cones that the sporangia are already M2 PRACTICAL BOTANY. open, dehiscence having taken place in a plane parallel to that of the leaf. II. Cut out as thick a piece of the stem as can be found, and about one inch in length : note on the surface of transverse section a central white dot this is the ; single central vascular bundle. Slice off the upper surface of the stem with a razor till the whole course of the vascular bundle is laid bare, and observe with a lens 1. The course of the central vascular bundle, which is directly longitudinal and median. 2. The smaller lateral bundles, which pass from the central bundle without branching, into the leaves, and traverse the midribs of the leaves. III. Cut transverse sections of a well developed stem : mount some in glycerine, others in Schulze's solution (others again may be mounted in acid solution of aniline sulphate). Examine first under a low power, using a high power when necessary, and observe the following tissues in succession, starting from the periphery of the section : 1. At the periphery a layer of small, thick-walled cells, forming an ill-defined Epidermis, with no stomata; it is covered externally by a continuous Cuticle. Beneath the epidermis, and not clearly marked off from it, is 2. The Cortical tissue : the cells- of the peripheral part of it have thick, stratified, and lignified walls, with no intercellular spaces. Passing inwards there is seen a gradual decrease in thickness of the walls, and increase in size of the cells, till an abrupt limit is reached at SELAGINELLA. SPOROPHORE. 165 3. The Lacunar tissue, consisting of thin-walled cells, which form irregular trabeculae traversing the intercellular cavity in a radial direction : the inmost cells of these trabeculae have a peculiar equatorial constriction. This lacunar tissue is more typically represented in some of the larger species, e.g. S. incequalifolia, S. Willdoncvii, &c. It may be regarded as the equivalent of the bundle-sheath, of most other vascular Cryptogams. 4. By means of these . trabeculse the single central Vascular bundle is suspended in the middle of the large air-cavity : the bundle is built upon the concentric type, and is composed of the following tissues : a. The Phloem-sheath, an irregular band of comparatively large, thin-walled cells, which completely surround the central tissues, and abut externally on the intercellular cavity, and the trabeculse. N.B. The cells of this layer, in common with all the outer tissues, including the epidermis, may contain chlorophyll granules, which are large, and only few are to be found in each cell. I. The Phloem, recognised as a tissue with thin cellulose walls, small cavities, and sparing protoplasmic contents ; it forms a continuous band surrounding c. The central Xylem, which appears as a spindle- shaped mass of tissue when seen in transverse section, and consists of elements with lignified walls, and no cell-contents. N.B. Small vascular bundles of rounded outline, as seen in the transverse section, may be found opposite or 166 PRACTICAL BOTANY. near to the ends of the spindle-like vascular bundle ; these are bundles of the leaf-trace cut through on their course inwards from the leaves. Thus the whole bundle - system of this shoot consists of a single central bundle, which traverses the axis longitudinally, and gives off smaller branch bundles, which pass outwards into the leaves, one of them entering each leaf. The vascular system of the shoot is more complicated in certain other species of Selaginella ; thus in S. Willdonovii, 8. incequa- lifolia, &c., three large flattened bundles are seen in each transverse section of the axis ; the planes in which these bundles are flattened are parallel to one another ; in these species the bundles may individually show considerable irregularities of out- line : the whole arrangement of their vascular system may with advantage be compared with that in the stem of Lycopodium. Note with a higher power : 1. The general appearance of the Phloem, with its highly refractive cellulose walls, and scanty protoplasm. 2. Between this and the xylem a somewhat irregular series of cells of the Conjunctive parenchyma, 1 with thin cellulose walls and plentiful protoplasm. 3. The chief constituents of the Xylem, viz., large prismatic Tracheides, with peculiarly marked, lignified walls. 4. At the poles of the spindle-shaped xylem note tracheides of smaller size ; these compose the first formed Protoxylem : development thus proceeds from the periphery to the centre. 1 It has been suggested by Treub that this term, which has been applied to the central parenchyma of certain roots, may be extended so as to include that parenchymatous tissue which has been termed by Russow "Geleitzelleo." SELAGINELLA. SPOROPHORE. 167 To confirm this, cut transverse sections of the stem about one inch from the apex, and treat as above. It will be seen that the elements near the poles of the xylem are already fully formed, and their walls lignified, while those at the centre are still thinwalled, and have protoplasmic contents. The lacunar tissue will be found to be better defined in these sections. IV. Cut longitudinal sections of a stem of S. Martensii. N.B. Since, owing to its being fixed in its place only by the weak trabeculse, the vascular bundle is apt to be detached in cutting accurately longitudinal sections, it will be found better to cut the sections slightly oblique ; it must then be remembered in examining them under the microscope that the sections are not truly longitudinal. Mount some in glycerine, others in Schulze's solution, and examine them under a high power, noting the same succession of tissues on starting from the outside, as were seen in the transverse sections : thus 1. Epidermis \ these are hardly to be distin- 2. Outer cortex J guished one from another ; the cells of both are prosenchymatous, and thick walled, and show a gradual transition to 3. The Inner cortex with thinner walls, and of parenchymatous shape. 4. The Lacunar tissue, in which may be distin- guished a. The outer parenchyma, consisting of short and small cells. b. The inner cells, which are elongated in a radial direction, and show the peculiar median constriction, above noted in the transverse sections. 5. The Phloem-sheath, consisting of elongated 168 PRACTICAL BOTANY. parenchymatous cells, with cellulose walls, and often containing chlorophyll. 6. The Phloem, the most prominent elements of which are long narrow structures with cellulose walls and sparing contents : these are regarded as the repre- sentatives of the Sieve-tubes. 7. The Xylem, the most prominent elements of which are spiral and scalariform Tracheides, similar to those composing the xylem of the bundle in the Ferns (see below, p. 195). The walls are lignified and thickened, and marked by elongated pits, which by their regular arrangement give the scalariform character to these elements. Y. Cut transverse sections of a Rhizophore, mount as before, and observe that (1) the peripheral tissues are not unlike those of the stem, and are marked off from the central cylinder by a somewhat irregular Bundle-sheath : (2) that the arrangement of tissues of the central cylinder differs both from that of the stem, and that usual for root-structures, there being but one group of Protoxylem (monarch), which is placed laterally, and the later formed Xylem forming together with it a central mass, which is surrounded by Phloem except at the point opposite the protoxylem. The structure of the individual vascular elements is similar to that in the stem. If successful median longitudinal sections be cut through the apex of a rhizophore it will be found that there is no root-cap. Further, by comparison of a number of sections, both longitudinal and transverse, it may be concluded that there is one apical cell having approximately the form of a quadrangular pyramid. VI. Cut transverse sections of a Root, and mount SELAGINELLA. SPOROPHORE. 169 as before : the structure will be found to resemble that of the rhizophore. If median longitudinal sections be cut through the apex of a root, a root-cap will be seen, which covers an apical cell, having the form of a triangular pyramid. VII. Mount some leaves, both dorsal and ventral, which have been previously bleached in alcohol, in water, or dilute glycerine : examine vdth a low power, and observe : 1. The difference in form of the dorsal and ventral leaves. 2. The central Midrib. 3. The ciliate margin. 4. The characters of the Epidermis ; thus on a. The upper surface the cells are small, and circular in surface view, with sinuous lateral walls, and no stomata ; &. The cells of the lower surface are elongated, with pointed ends, and sinuous lateral walls ; over the midrib the cells are shorter, and it is there only that the Stomata are found, having two guard-cells, and no subsidiary cells. Large chlorophyll grains are to be found in the cells of both the upper and the lower epidermis, and in these the included starch-grains may often be well seen after treatment with iodine. VIII. Cut transverse sections of fresh leaves held in a piece of pith : mount in water or weak glycerine, and observe : 1. The Epidermis of the upper surface (without 170 PRACTICAL BOTANY. stomata) consists of conical cells, with few, very large chlorophyll granules. 2. Beneath this is the Spongy parenchyma, which encloses centrally A 3. single Vascular bundle. 4. The Epidermis of the lower surface consists of smaller cells containing chlorophyll, and with Stomata opposite the midrib : note the two small guard-cells as seen in transverse section. Near the margin of the leaf the upper and lower epidermal layers are in contact with one another, the spongy parenchyma being there absent. There is also a marginal band of thickened cells. IX. Choose out from material which has been hardened in alcohol the apical buds of branches which have not as yet begun to form sporangia : holding these between pieces of pith, or carrot, or otherwise embed- ding them, cut longitudinal sections ; mount them in glycerine, examine first with a low power, and select those sections which are nearest to the median plane (i.e. those which show the greatest regularity of parts, and the stem terminated by the apical cone). In such sections observe : 1. The Axis with tissues as above described (cf. longitudinal sections), and terminated by the apical cone : borne laterally on this are 2. The Leaves, each having a Ligule attached to its upper surface : note also their structure as above described. Passing towards the apex of the bud observe successively earlier stages of their development. Examine the sections with a higher power, and observe : SELAGINELLA. SPOROPHORE. 171 1. The arrangement of the cells at the summit of the apical cone, which is terminated by an apical cell ; from this segmental cells are successively cut off. 2. The origin of the leaves is not from a single cell, but by the outgrowth and subsequent division of a number of cells at the periphery of the apical cone. In such median longitudinal sections should also be observed the differentiation of the vascular bundle from the primary meristem, and also the development of the lacunar tissue, and its relation to the central bundle. Preparations may also be made of the apex so as to show the structure of the apical cone as seen from above. By comparison of a number of these it may be seen that the form of the apical cell is by no means constant, but varies between the forms of a two-sided and a three-sided cone. X. Cut longitudinal sections through fertile branches similar to those cut from the vegetative bud, and examine them under a low power. Observe that the general arrangement of the stem, leaves, and ligules is the same as in the vegetative bud. In the lower part of the sections a mature Sporan- gium may be found in the axil of each leaf. (The sporangium may have lost its spores partially or entirely during the preparation of the sections.) It will consist of A (a.) short massive Stalk. A (&.) Wall enclosing the central cavity : the wall will be found under a high power to consist of three layers of cells i. The outer consisting of thick-walled cells, more or less elongated radially. 172 PRACTICAL BOTANY. A ii. layer of small, compressed cells. A iii. layer of thin-walled cells, elongated radially : this is the Tapetum, which is here persistent until the spores are ripe. Surrounded by the wall will be found (c.) Spores of two sorts i. Microspores of relatively small size; these will be found in large numbers in certain sporangia, which will accordingly be recognised as Microsporangia. When ripe they may be still seen to cohere in groups of four : each spore is a single cell with a brown wall. ii. Macro spores of relatively large size : four only of these will be found enclosed in the sporangium, which is accordingly termed a Macrosporangium. Each spore consists of a thick wall, with numerous external projections, surrounding a large cavity filled with protoplasm. The development of tlie sporangium may be traced in longi- tudinal sections of sporangium-forming cones which have been hardened in alcohol, or better, in picric acid and then in alcohol ; mount in glycerine. The following points in the process of development may be observed. The sporangium is first seen as a swelling of a group of cells at the surface of the apical cone, above the leaf in the axil of which it appears : thus the sporan- gium is not borne on the leat as in Lycopodium, but springs A from the tissues of the axis. central row of cells grows more strongly than the rest, and the outermost cell but one of this series may be recognised as the Archesporium. The outermost cell divides to form part of the two outer layers of the wall of the sporangium. The archesporium also divides to form a mass of tissue, of which the peripheral layer becomes the Tapetum (the basal part of the tapetum is however derived from the adjoining tissue). The central part of the tissue derived from the archesporium forms the spores ; each spore-mother-cell sepa- rates from its neighbours, and divides into four cells. If the spo- SELAGINELLA. OOPHORE. 173 rangium is to develop macrospores, only one of these groups of four cells is further developed, the rest being abortive ; if it develop microspores, all the groups of four are further de- veloped, but only attain a comparatively small size : in both cases the four spores may separate from one another when quite mature, though they often retain their original arrangement. THE OOPHORE. XI. Spores of both kinds may be obtained free by drying branches which bear sporangia on sheets of paper. Pick out the macrospores, and mount them in olive oil; dissect off the brittle outer coat of the spore with needles, and examine under a high power. It will be seen that the chief contents of the ripe spore are a protoplasmic matrix enclosing oil globules and aleurone grains, while traces of the cells of the Pro- thallium may be recognised even in these preparations. Prepare other such spores with potash, and dissect as before, or press on the cover slip, and warm gently. It will be found, when the oil, &c., has been acted upon by the potash, that a part of the contents of the spore is traversed by a distinct network of cell-walls, forming a meniscus-shaped mass of tissue. If plenty of spores are to be had, it will be found better to embed a quantity of them in cocoa-butter, and to cut sections, and mount them in glycerine. Observe 1. The character of the wall, consisting of a. An outer thick, yellow Exospore. &. An inner thin Endospore. 2. The contents as above described : the natural position of the cellular tissue of the Prothallium may be seen to be at the apex of the cavity of the spore. 174 PRACTICAL BOTANY. XII. Spores of both kinds should be collected in considerable quantity by drying on paper, and then be sown on moist soil or sand, and left to germinate. In a few weeks young seedlings will be seen with an erect axis, bearing small leaves. The axis branches at an early period. Remove one of these seedlings from the soil, and note the bifurcations of the root, and the macrospore still attached laterally to the axis. By careful comparison of spores thus sown, it may be observed at various times during their development that the contents of the microspores divide into a number of cells, and ultimately rupture and set free antherozoids : also that the tissue of the prothallus in the macrospore increases, rupturing the wall of the spore, that archegonia are formed, from one of which the young seedling originates: for further details the Text-books must be consulted. II. LYCOPODIUM CLAVATUM (The Common Club-Moss). SPOROPHORE. I. In a well-grown plant recognise the following external characters : 1. The Stem, often extended to a great length, is creeping, and frequently branched, apparently in a monopodial manner (for particulars see below) : the stronger branches are also creeping, the weaker branches ascending. 2. From the under side of the stem Roots are de- veloped, which frequently (but not always) appear at points where the stem branches. The roots themselves are branched dichotomously, but the limbs of the dichotomy may develop either equally or unequally. 3. The stem is covered with Leaves, which are simple in form and linear, with ciliate margin, and a long awn-like apex. The arrangement of the leaves is complicated, and has been described by Braun as being partly in whorls, partly spiral : the number of members of the whoils is variable, as is also the angle of divergence of the spirally-arranged leaves. 4. The fertile branches, or Cones, which bear 176 PRACTICAL BOTANY. sporangia, are erect and elongated ; their lower part is covered sparsely with leaves of small size : about 1 2 inches below the apex they usually divide into two or three branches, covered with rather broader, closely imbricated leaves : from the upper surface of each of these rises one Sporangium, which is yellow when ripe, and opens by a split parallel to the plane of the leaf which bears it. II. Cut transverse sections of a fully developed stem. Mount some of them in glycerine, others in Schulze's solution, and examine with a low power: externally will be found 1. An Epidermis, consisting of a single layer of cells; their outer walls are thick, and covered by a continuous layer of Cuticle, which may be recognised in thin sections by its high refractive power. 2. Below the epidermis lies a broad band of Cortical' tissue, which appears differentiated into successive thinner bands according to the thickness of the cell- walls : thus there may be distinguished a. An external sclerenchymatous band, with thick lignified walls (brown with Schulze's solution) ; small intercellular spaces may be seen at the angles between the cells : these cells retain a small proportion of their cell-contents. b. Within this is a broad band of thin-walled tissue, in which the cell-contents are not apparent : the cell-walls are tinged with pink in Schulze's solution. There is a gradual transition from this to c. The most central part of the cortex, which is strongly sclerenchymatous; it has intercellular spaces, and retains its cell-contents. It forms a dense LYCOPODIUM. SPOROPHORE. 177 band of lignified tissue (brown with Schulze's solution). Here and there may be found in the cortex small groups of very narrow elements, having a dark appear- ance : these are single Vascular bundles of the leaf-trace, cut through on their course from the A leaves to the central vascular cylinder. sudden transition is found from the inner sclerenchymatous band (c) of the cortex to 3. The tissue, which lies next to it internally. This tissue consists of two to three layers of tangentially elongated cells, the walls of which have a sharp con- tour, are not thick, and stain with Schulze's solution a slightly different tint of brown from the walls of the sclerenchyma : this band is regarded as taking the place of the Bundle-sheath, which is met with in most of the Pteridophyta as a definite layer of cells. Treat a section with sulphuric acid : the walls of this band will be found to resist* its action more than the rest of the tissues, the walls being of a corky nature. Note also that the cuticle is brought into greater prominence by treatment with sulphuric acid, since it resists the action more strongly than the other walls. Within this so-called bundle-sheath is found A 4. cylindrical mass of vascular tissue. It is composed, as in other cases, of (a) Phloem tissues, and (&) Xylem tissues, which may be distinguished by their optical properties, and by their staining with various reagents. Observe that the phloem forms a matrix, as it were, in which are embedded the masses of xylem ; the latter are of elliptical form as seen in the transverse section : several of these N 178 PRACTICAL BOTANY. are ranged side by side, the longer axes of the ellipses being horizontal as the plant grows, and parallel to one another : the masses of xylem may be irregularly connected one with another towards the centre of the stem. III. Before proceeding to the study of these several tissues in detail, cut transverse sections from the young stem, at about one eighth of an inch below the apex ; treat some of these with dilute potash, others with Schulze's solution. Observe that in these sections the tissues at the centre of the vascular cylinder are still thin-walled, and have plentiful protoplasm, i.e., they are not fully developed ; towards the periphery, however, will be seen a series of groups of tissue showing the characters of developed xylem, and alternating with these is a series of groups of phloem tissues. Other preparations may be made successively from points further from the apex, and from these the conclusion may be drawn that the vascular tissues at the periphery of the vascular cylinder are matured first, and that the development proceeds towards the centre. (Compare roots.) IY. Returning to the sections of the old and mature stem, examine the vascular tissues under a high power. i. Immediately within the so-called Bundle-sheath is a band of tissue, which abuts directly upon the periphery of the xylem and phloem, and having cell- walls which stain blue with Schulze's solution; this may be regarded as the Phloem-sheath. On ii. examining the masses of Xylem, observe that a. The constituent elements are much smaller at the periphery of the vascular cylinder than towards the centre the former ; are the first developed or Proto- xylem elements. &. The main constituents of the xylem are elements with large cavity, and of rounded polygonal form LYCOPODIUM. SPOROPHORE. 179 (Tracheides) ; note the structure of the wall, especially where two of these adjoin one another. iii. In the phloem there will be found at the periphery of the vascular cylinder, and alternating between the successive groups of protoxylem, masses of tissue with thick cellulose walls, and small cell-cavities ; these are the Protophloem groups, or first formed elements of the phloem. Passing from these towards the centre of the vascular cylinder, the phloem is found to consist of a. Constituents with large cavities, and very scanty cell-contents. ~b. Elements with small cavity, and obvious cell- contents. V. Cut radial longitudinal sections through a mature stem : mount as before, examine them first with a low power, and note 1. The bases of the leaves continuous with the epidermis and cortex of the stem. 2. The Cortex showing the same differentiation into successive bands (a), (&), (c), as was seen in the trans- verse section, 3. The central Vascular cylinder. 4. Small Vascular bundles of the leaf-trace, which may be seen pursuing an oblique downward course from the bases of the leaves, through the cortex to the periphery of the vascular cylinder ; since these bundles may be followed in one radial section from the leaf to the central cylinder, it follows that that part of their course is approximately in a radial plane. We can now obtain a clear idea of the Vascular System of a mature shoot of Lycopodium cfavatum : 180 PRACTICAL BOTANY. there is in the first place a central vascular cylinder, which traverses the shoot longitudinally, and from the periphery of it single bundles of small size are given off, which take an obliquely ascending course in radial planes, and each of them enters a leaf. By further comparison of longitudinal and transverse sections it may be ascertained that the masses of xylem in the central cylinder have the form of flattened plates, the planes of which are approximately horizontal in the living plant. They are sometimes separate from one another, sometimes joined towards the centre of the stem, and it is on the margins of these plates that the bundles of the leaf-trace are inserted. Examine the radial longitudinal sections under a high power, and observe i. That the cells of the epidermis, and of the sclerenchymatous portion of the cortex are elongated and prosenchymatous, while those of the thin-walled band of the cortex are shorter, and tend to a paren- chymatous form. The walls of the cortical cells are pitted. ii. Of the vascular tissues the Xylem is the most prominent ; its chief constituents are of prosenchymatous form, with lignified walls : the latter show the scalariform marking, which is due to the regular arrangement of elongated pits with their longer axes placed horizontally : each of these pits shows a double contour, and transitional forms will be found from the elongated to circular pits, the latter presenting an appearance very similar to that of the bordered pits of Pinus from which the elongated pits differ only in their outline as seen in surface view. Some of the LYCOPODIUM. SPOROPHORE. 181 walls will have been cut through longitudinally : examine a section of one of them, and note especially that the pit-membrane is constantly present ; there is thus no communication between the cavities of these elements, and they have no cell- contents remaining; they are therefore scalariform tracheides (compare those in the xylem of the Fern, and the tracheides of Pinus.) iii. Where the sections have passed through the peripheral margins of the plates of xylem, there will be found elements of the Protoxylem, which correspond in structure to those in the stem of Pinus : irregular finger-like outgrowths of the cell-wall may be observed extending into the cavity of these elements. iv. The phloem, intervening between the masses of xylem consists of a. Prosenchymatous cells with cellulose walls, and granular cell-contents; these are directly in contact with the xylem. b. Long tubular structures, the pointed endings of which are very rarely met with ; their course may be followed for a considerable distance in a longitudinal direction ; they have transparent contents, and their cellulose walls are dotted with minute pits, about which bright globules adhere. These are probably the representatives of the Sieve-tubes of the Phanerogams. Stems of other species of Lycopodium may be treated in the same way, and a comparison made of their structure ; the general arrangement of tissues will be found to be fundamentally the same as that described for Lycopodium clavatum, the differences depending chiefly upon the number of plates of xylem and phloem, and variations in the manner and extent of the connection between the plates of xylem. 182 PRACTICAL BOTANY. VI. Cut transverse sections of the leaf; this may easily be done either by embedding the whole stem with the leaves attached in paraffin or cocoabutter, cutting transverse section of the whole, and then picking out the sections of the leaves; or by holding the stem with the leaves between the finger and thumb, and cutting transverse sections from the whole as from a solid mass. Mount as before examine ; under a low power, and note 1. The outline of the sections roughly triangular. 2. The single layer of Epidermis with circularised outer wall : Stomata are found both on the lower and the upper surface. 3. Beneath this is the Mesophyll, with large in- tercellular spaces : the cells, which form an irregular network, are nearly globular, have thin walls, and contain chlorophyll granules. 4. At the centre is a single very small Vascular bundle. VII. Cut median longitudinal sections through the bud : use material which has been preserved in alcohol, or hardened in picric acid and then in alcohol. Mount in glycerine, and examine with a low power : note that at the lower part of the section the central vascular cylinder will be easily recognised, while the bundles from successive leaves pass obliquely through the cortex, and insert themselves upon its margin. Passing upwards, however, towards the apex, it gradually loses its dark appearance (due to developed xylem ) ; still its continuity may be traced up to the apical cone, as a bright-looking strand of formative tissue consisting chiefly of prosenchymatous LYCOPODIUM. SPOROPHORE. 183 elements (Plerome or Procambium), while the forma- tive tissue of the cortex external to it is more typically parenchymatous (Periblem) ; it is limited by a not very definite layer of cells which may be recognised as the Dermatogen. The conical apex itself consists of a dome-shaped mass of meristem; the layer of dermatogen, which may be recognised at the base of the cone, may be followed up nearly to the apex, but there loses its identity, the extreme apex being occupied, at least in the more bulky examples, by a group of initial cells, which divide by anticlinal walls ; those at the margin of the group divide also periclinally. Compare this on the one hand with the structure of the apex of the stem of Phanerogams, and on the other with that seen in the Ferns. Observe further that the development of the leaves begins by the outgrowth and division (both anticlinal and periclinal) of groups of cells, which constitute multicellular protuberances; these have at first an apical growth, which soon ceases, the further growth being basal and intercalary. The origin of the branches may further be observed in these preparations ; it will be seen that they arise in this species below the apex of the main axis, and laterally upon it ; the branching is thus monopodial not dichotomous. VIII. Cut transverse sections of one of the thick roots: mount as before, and observe that they resemble the transverse sections of the stem in the arrangement of the tissues, though the whole structure is simpler : there are usually only three plates of 184 PRACTICAL BOTANY. xylem; these are often complicated by irregularities, e.g.y fusion, &c., and are less strongly developed in their central portion than those of the stem. If similar sections be cut successively from roots of higher order, they will be found to show successively reduced types of structure, till the xylem is finally represented only by a single group of elements, which is surrounded by tissues of the phloem. Median longitudinal sections may be cut through the apices of roots which have been hardened in alcohol : from these it will be seen that there is a stratified structure of the apical meristem, in which may be recognised a distinct root-cap, marked off from the layers below it by a layer of dermatogen, which can be traced as a continuous layer for a considerable distance beneath the root-cap : centrally a strand of plerome may be recognised. Bifurcations may be found in such sections, showing that the branching is a true dichotomy. IX. Cut median longitudinal sections through a cone, bearing Mature sporangia : mount in glycerine, and examine under a low power ; observe the structure of the axis as before seen in longitudinal section, with a vascular system sending out branches into the leaves ; the chief difference between this and the vegetative axes is the presence of sporangia. Note i. That one sporangium is seated with a short stalk on the upper surface of each leaf. ii. That no branch of the vascular system enters the stalk of the sporangium. iii. That the cavity of the sporangium is surrounded by a thin wall. iv. That the cavity thus inclosed may be