Part II

mediately within the corky rind) : treat the sections for a few minutes in alcohol with a few drops of iodine solution added or ; SUGAR 221 in very slightly diluted tincture of iodine, and mount in pure glycerine : examine under a high power, and observe 1. The cells of the usual parenchymatous type, with protoplasm and nucleus. 2. Numerous spherical bodies, usually aggregated round the nucleus : in some cells these will stain blue (starch), with a small yellowish body attached which is the leukoplast ; in other cells, which were nearer the outer surface of the tuber, the bodies will stain uniformly yellowish brown : these are the young leilkoplasts which have not yet formed starch. The material which has been found best suited for the observa- tion of leukoplasts is the young tuber of the Orchid Phajus grandifolius (Bletia TanJcervillice), but it is not to be expected that this will be within the reach of all. Here the leukoplast is of large size, and rod-like form : this material may be treated similarly to the Potato, or a method recommended by Strasburger may be adopted, viz., to cut the sections and fix and stain them with picro-nigrosin, which colours the leukoplasts a steel-blue tint. II. For inulin and its reactions see p. 119. III. Cellulose occurs as a reserve in the endosperm of the Date, and other Palms : the appearance of these has been described on p. 38, and the sections should be cut and the reactions noted. Thickened cell-walls will also be found in sections of the cotyledons of Lupinus. IV. Sugars. (a) Grape-sugar. Cut a transverse section of a ripe grape, of such thickness that some cells at least shall be uninjured : mount in water, and observe under a low power the transparent parenchymatous pulp, consisting of cells with thin walls, very sparing contents, and large central vacuole. Treat for a few minutes with a relatively large bulk of alcohol in a watch-glass : on re-examining, numerous crystals will now be seen in the cells. 222 PRACTICAL BOTANY Irrigate thoroughly with water : the crystals may be seen to be re-dissolved : they consist of grape-sugar, which is in solution in the cell-sap of the living cell. 2. Squeeze out the juice of some Grapes into a testtube : add a little of Fehling's solution (see Appendix A), and boil : a bulky yellow precipitate is formed owing to reduction of the copper. 3. Soak a fairly thin section of a Grape in Fehling's solution : wash quickly with water, mount in water, and boil gently over a spirit-lamp : a precipitate like the above (2) is formed : note under the microscope that the dark-looking granules of the precipitate (cuprous oxide) are to be found actually within the cells of the tissue, thus indicating that the sugar was there. (b) Cane-sugar. 1. Cut transverse sections of the Beet-root : mount them in water, and note under a low power the transparent tissue, and coloured cell-sap. Treat such a section for a few minutes with alcohol in a watch-glass : on re-examination under the microscope, crystals will be seen in the cells, but of smaller size than in the Grape. Re-dissolve by irrigation with water. 2. Boil some small pieces of Beet-root in a smalLbulk of water : pour off the coloured extract, add to it a little of Fehling's solution, and boil : no precipitate will be formed. 3. This point may be further verified by testing sec- tions as directed above, under head (3) : no precipitate will be formed either in the cells or in the surrounding fluid. V. Oil-drops. Cut thin sections of the cotyledons of the Almond ; OIL-DROPS, ETC. 223 mount in water, and note the bright-looking oil- drops, both in and about the section, and dispersed also in the water. 1. Irrigate with alcohol : the drops are not dissolved. If sections of the oily endosperm of Ricinus be treated thus, the oil will be dissolved : this is characteristic of ethereal oils, but is as a rule not the case with the fixed oils : to this the oil of Ricinus is an exception. 2. Treat a section with a considerable bulk of ether in a watch-glass : wash with alcohol, and mount in alcohol, or in glycerine : on examination the oil will be found to have been dissolved by the ether. 3. Stain a thin section with tincture of alkanet (see Appendix A), the oil-globules stain pink. 4. Treat a section with 1 per cent, solution of osmic acid : the oil-drops will stain slowly, taking a dusky or black hue. 5. Treat a section with potash solution, and warm : the oil will be partially and slowly saponified and dissolved. This effect of potash is best seen in specimens where the oil is present only in small quantities as isolated globules. VI. Aleurone-grains. These are found of the largest size in oily seeds : they vary somewhat in their characters in different plants : those of Ricinus may be taken as a good type. Having noted the hard, variously marked testa, with the wart-like swelling (aril) at the basal end, crack and remove it: beneath it will be found the white oily mass of the endosperm. Cut this through transversely, and with a razor wetted with olive oil, or castor oil, 224 PRACTICAL BOTANY cut thin sections from it, and mount in the oil. Examine under a high power, and observe 1. The thin cell-walls of the oval cells. 2. The numerous highly refractive aleurone-grains in each cell ; each is of oval form, and a less highly refractive area is seen at one end: this is the globoid. 3. The oily matrix in which the grains are em- bedded, this being so transparent as to be hardly visible. Cut other sections with the razor wetted with alcohol, soak them well in alcohol in a watch-glass to dissolve the oil out of them (ether is a more ready solvent of the oil, and may be used instead of alcohol : wash off the ether with alcohol), and mount in pure glycerine : examine them under a high power, and observe the appearance of the aleurone-grains as before when seen in oil. i. Add water gradually, and watch its effect on the grains. 1. The outer amorphous coat of the grain will swell, and become less highly refractive : thus there will be disclosed 2. The crystalloids, one (or rarely more) being included in each grain : these do not swell greatly with water, and accordingly they retain their refractive power. 3. The globoid will also be visible as before. ii. Add dilute potash solution : the amorphous coats and the crystalloids will swell and dissolve, leaving the globoids. iii. Add strong acetic acid : the globoids will dissolve GERMINATION 225 slowly. As regards further reactions of aleurone-grains see Appendix B. VII. Crystalloids. Crystalloids of cubical form are to be found in the superficial tissues of the Potato. Cut tangential sections from material hardened in alcohol, or in picric acid and alcohol, and mount in pure glycerine and iodine : the cubical crystalloids will be distinguished by their yellowish brown staining. Treat a section, in which one or more crystalloids are under observation, with potash : the crystalloids will be seen to swell and dissolve. Mount another section in alcohol or in glycerine, and, having observed the crystalloids, irrigate with a saturated solution of common salt : this also will dis- solve the crystalloids. GERMINATION. (A) Dicotyledons. (a) Examine seedlings of Helianthus which have been germinating for different periods from one day to one week, and observe the following points in the process of germination : 1. The internal parts of the fruit swell, and cause the brittle pericarp to split longitudinally. 2. The radicle protrudes, and curves downwards. 3. The hypocotyledonary stem elongates, so that the pericarp and seed-coat are carried upwards by the cotyledons, which remain inclosed by them for a considerable time. 4. The coats of the fruit fall from the cotyledons, Q 226 PRACTICAL BOTANY which soon turn green, and expand as assimilating leaves, with the plumule seated between them. 5. The plumule develops leaves, which expand in succession. 6. The radicle has meanwhile elongated, and produced lateral roots. Notice that when the young root is removed from the soil many particles adhere to it, especially at some distance from the apex : these are held by the root- hairs which attach themselves closely to the particles of soil. (1) With the above compare seedlings of Ricinus in various stages of germination : in the main features the results are the same, but note especially that the endosperm remains for a long period in close connection with the cotyledons, and that as the seedling grows that tissue loses its firmness and density, owing to the abstraction of the nutritive substances stored in it, and their transfer through the cotyledons to the seedling. (c) It will be found useful to compare the germina- tion of other seeds also, e.y. the Broad Bean, Kidney Bean, Cucumber or Gourd, &c. The internal changes accompanying the process of germination, and more especially the redistribution of the reserve materials stored in the embryo, may be studied by cutting sections of the seedling at different stages of the process, and comparing the cell-contents in the corresponding tissues. Note especially the corrosion of the starch -grains, in those cases where starch is stored in the seed. MAIZE GERMINATION 227 (B) Monocotyledons. I. Comparing plants of Maize which have been germinating for different periods, the following facts in the history of germination may be observed : 1. The fruit swells. 2. The outer coat ruptures opposite the apex of the radicle, which soon protrudes, bursting through the coleorhiza also, which appears as an irregular ring round the base of the young root. Since the coleorhiza is thus burst through by the young, root, it is clear that the epidermis of the shoot is not continuous with the piliferous layer of the root. 3. The rupture of the coat extends upwards to the point opposite the apical bud, which also emerges. 4. The root elongates, and forms lateral roots : other lateral roots (usually two) burst out above the insertion of the scutellum : these soon equal the primary root in length, hence there is no well marked tap-root. 5. Leaves of the plumule unfold, and gradually turn green : the leaf inserted lowest, which was the outermost of those composing the plumule, remains small and develops no expanded lamina : this is the cotyledon, according to Hofmeister and other writers. II. From a young plant with leaves about three inches long, cut longitudinal sections so as to traverse the whole fruit and the contiguous part of the seedling in a median plane : mount in water, and irrigate with solution of iodine. Observe 1. That in the neighbourhood of the surface of the Q2 228 PRACTICAL BOTANY scutellum the starch-grains are in course of demolition, and that the central part of each is first attacked. 2. That no starch-grains are to be seen in the epithelium of the scutellum. Seeds of the Date should also be sown, and the process of germination followed : it may then be seen that the cotyledon, which remains in contact with the endosperm, exhausts the substance of it, gradually increasing the cavity in which the embryo originally was : meanwhile the body of the embryo, with plumule and radicle, is pushed out from the cavity, owing to the increase in length of the cotyledon. Asparagin. This substance is found in most seedlings, but in specially large quantity in those of Lupinus lutcus. Cut sections from fresh seedlings, and, after a preliminary examination of them, irrigate gradually with alcohol : the substance is precipitated in the form of crystals. Sections of material which has been kept in alcohol, mounted in glycerine, will show these crystals already formed, and often of large size. These sections should be irrigated with water ; the crystals will dissolve. The most distinctive test is a saturated solution of asparagin : if this be added to sections containing crystals presumably of asparagin, they will not be dissolved, whereas crystals of inulin would be dissolved. II. GYMNOSPERMS. VEGETATIVE ORGANS. EXTERNAL CHARACTERS. A. Examine a plant of the common Norway Spruce Fir (Abies excelsa), and observe that while the main axis grows vertically upwards, and bears pseudo-whorls of lateral branches, which radiate from it as a centre, the branches themselves are ramified only in a horizontal plane : the branches are thus dorsiventral, having distinct upper and lower surfaces. The branches as well as the main axis bear numerous simple, linear, four-angled foliage-leaves, which may however have dropped otf from the older parts, leaving clearly-marked scars. Note also at the bases of the lateral branchlets, and at irregular distances along the relative main branch or the main axis itself, that there are zones where dry chaffy scale-leaves were inserted : these protected the winter-buds when young, and their bases remain persistent : the portions between successive zones represent the annual increments of growth in length, and, as the lateral branches are for the most part inserted immediately below these zones of scales, 230 PRACTICAL BOTANY it follows that they are produced at the upper end of each annual increment. Examine the buds at the apices of the stem and branches : they are covered with brown scale-leaves : thus a distinction is to be drawn between protective scale-leaves, and the foliage-leaves. There is considerable variety in the character of the nonreproductive leaves of the Coniferw, though the form of the individual leaf is simple throughout the series. In some cases only green foliage-leaves are developed, as in A raucaria, Juni- perus, Thuja; in Phyllocladus only scale-leaves are produced, while the flattened stems assume the assimilative function in ; A other cases, as in Taxus and lies, there is an alternation of scale- leaves which protect the bud, and foliage-leaves ; in Pinus sylvestris, the somewhat complicated shoot of which will be described in detail below, there are both scale- arid foliage-leaves, the former alone being borne on the stronger axis of unlimited growth, while the latter (together with scale-leaves) occur only on the foliage -shoots of limited growth (bifoliar spurs). Specimens of different genera of the Coniferce should be examined and compared. B. Take a branch of Pinus sylvestris, cut in autumn, including at least four years' growth. N.B. The limits of each year's growth may be recognized externally at those points where false whorls of strong lateral axes are developed ; and the portion of stem lying between two such whorls may be regarded as roughly representing one year's growth. T. 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 PINE VEGETATIVE ORGANS 231 remove the external secretion of the resin, 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 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 internode. 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 (bifoliar spurs), each bearing two acicular foliage-leaves, surrounded at the base with numerous scale-leaves. These bifoliar spurs occur in the axils of the scales throughout the greater part of the current year's growth : in older parts they may be found to have fallen off, the bifoliar spurs separating as a whole from the parent branch. I. Buds with unlimited growth, which are seated 232 PKACTICAL BOTANY 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 (6) 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). II. Passing to the increments of growth of formei1 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 bifoliar spurs drop off, leaving scars which mark their former position. 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 chlorzinc-iodine : examine with a 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 chlor-zinc- iodine) : surrounding this a series of groups of smaller constituents : these are 2. The primary vascular bundles. Note that they PINE STEM 233 are separated from one another laterally by bands of parenchyma ; that their form is approximately wedgeshaped ; and that the tissues of which they are composed may be distinguished as i. Xylem, nearer the centre of the stem, the com- ponents of which have thick, dark-looking, lignified walls (yellow with chlor-zinc-iodine). These first-formed xylem-elements, since they differ from those formed later, are distinguished as protoxylem. ii. Phloem, nearer the periphery, with bright-looking cellulose walls (blue with chlor-zinc-iodine). 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 chlor-zinciodine : the sections have a wavy outline, the inden- tations 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, are much thickened : externally there is a cuticle. 2. Cortical tissue, consisting of cells with rather thick cellulose walls (blue with chlor-zinc-iodine), and 234 PRACTICAL BOTANY 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. (Compare p. 65). 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. The secretion stains deeply with tincture of alkanet. (See p. 65.) Near the periphery of the cortex will be found a layer of cork and a cork-cambium (compare stem of Elm, p. 91), derived from cells of the cortex by their division by tangential walls. The cells of the cork have no cellcontents : their walls are coloured yellowish brown with chlor-zinc-iodine. Treat a section with strong sulphuric acid. The walls of the cork retain their sharp contour. At the bases of the indentations at the margin of the section, and immediately below the epidermis, note groups of sclerenchyma, having thick lignified walls (yellow with chlor-zinc-iodine). 3. The vascular system is here a complete ring, though it is composed of separate bundles in the bud (see above, p. 232) : distinguish as before, the external phloem, the internal xylem, and the misty layer of cambium. The vascular bundles were seen to be separated in PINE STEM 285 the bud by intervening parenchyma : here the ring has been completed by the formation of an interfascicular cambium in the parenchyma between the original bundles. (Compare p. 99.) Observe that the internal limit of the vascular ring is sinuous : the convexities mark the position of the primary bundles : at the inner limit of these will be found the protoxylem. 4. The pith consists of parenchyma, having the same characters as in the bud : there are 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 chlor-zinc-iodine). iii. That the tangential walls are thinner than the radial. iv. That the cells have copious protoplasm, in which a nucleus may often be recognized. These facts point to a repeated division of cells by tangential walls. (Compare Fig. 9, A, p. 99.) 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 observa- tions 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 236 PRACTICAL BOTANY and inwards towards the pith : they are the medullary rays. (Compare Fig. 9, A, row 2.) Some of them may be traced the whole way to the cortex and to the pith (primary medullary rays), 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 xylern or phloem, the cambium being the formative region of the rays as well as of the other tissues. The mature cells of the ray usually have cellulose walls (blue with chlor-zinc-iodine), and granular proto- plasmic 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 chlor-zinc-iodine). The ring of secondary phloem is cut up into rectangular areas by the medullary rays, which are easily recognized 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