Vascular Tissue Structure and Function

bundles and connecting the cortex with the medulla are the medullary rays (iti). The vascular bundles, together with the medullary rays and pith, form a cylindrical mass of tissues known as the vascular cylinder or side, which extends continuously throughout the plant from the tip of the stem to the growing-point of the root. (i) The epidermis is usually one cell thick and acts as a pro- tective coat for the plant, preventing the latter from too rapid loss of water and also defending the deUcate internal cells of the plant against mechanical injuries due to rain, hail, frost and insect attacks. The cells are tubular flattened cells fitting quite closely together, except where the openings named stomata occur : as the latter are more abundant in the epidermis of a leaf, their structure is deferred to page 145. Usually the outer cell-wall of each epidermal cell is much thicker than the lateral and inner walls, and is differentiated into two or three layers, the outermost layer in contact with the atmosphere being spoken of as the cuticle. The cuticle is composed of a substance known as cuiose, which is very impervious to water, and a remarkably stable body capable of resisting the action of various solvents which dissolve ordinary cellulose. On the cuticle of the stems and leaves of cabbages, swedes, and many varieties of cereal and other grasses, as well as on grapes and plums, an ash-coloured bloom is seen. It is an excreted product of the epidermal cells, and consists of minute round, rod-like or scaly particles of wax. Surfaces of the different parts of plants covered with this bloom lose less water than those from which the substance has been removed by rubbing. This waxy layer appears also to act as a partial protection against the attacks of fungi and insects. The cells of the epidermis contain the usual cell-contents with the exception of chloroplasts which are generally missing f CORTEX tts they are especially rich in cell-sap, which is often tinted pink, red or purple by a colouring matter which appears to protect the cells of the cortex from excessive light. In some plants, if not in all, the cell-sap of the epidermal cells functions as a store of reserve water upon which the more internal cells of the stem can draw in time of need. It is well known that the surface of stems and other parts of plants are frequently covered with hairs. These belong to the epidermis, and in their simplest form are merely single cells which have grown much longer than their neighbours. Some hairs are, however, multicellular extensions of the epidermis (h, Fig. 54), and like the unicellular hairs may assume a great variety of shapes. Hairs are often harsh to the touch, and furnish a means of defence against insects and animals generally. They also act as a mantle which prevents too rapid escape of water from the plant, and acts as a screen against excessively bright sunshine. In young stems and buds, hairs protect the tender parts against injury by frost. Certain hairs function as secreting organs, and are then designated glands (Fig. 106) : they often produce resinous and oily compounds, which in the case of mint, hop, and other plants have a characteristic odour. Many excreted products of such hairs are sticky, and effectually prevent insects such as ants from climbing up the stem and getting at the nectar of the flower. (ii) The cortex of the stem extends from the epidermis to A the vascular cylinder. great part of it generally consists of living parenchymatous cells which contain abundant chloroplasts. The cells of the portion immediately beneath the epidermis frequently have their cell-walls thickened at the corners, and form what is spoken of as collenchymatous tissue : the latter serves to strengthen the epidermis, and gives rigidity to the whole stem. The innermost layer of cells belonging to the cortex forms a continuous sheath surrounding the vascular cylinder Ii6 ANATOMY OP STEM, ROOT AND LEAF termed the endodermis {en, Fig. 54) \ its cells are not very much differentiated from the rest of the neighbouring cortical cells, but they usually contain numbers of starch-grains which render them somewhat conspicuous in sections of certain stems. (iii) The vascular cylinder or stele includes all the tissues inside the endodermis, namely, the vascular bundles described below, and also the medulla or pith and the medullary rays. -- Fig. 54. Transverse section of the stem of a sunflower (enlarged about 8 diameters.) X^ portion including a vascular bundle; e epi- w dermis ; /( a hair ; c cortex ; en endodermis ; wood ; b bast ; fc fascicular cambium ; ic interfascicular cambium ; / pericycle fibres. The outermost portion of the stele which lies immediately in contact with the endodermis is known as the pericycle. The latter may consist of a single layer of cells or of more than one layer ; in some stems, its cells are thin-walled, and from it arise most adventitious roots and shoots. The medullary rays and pith are composed of thin-walled parenchymatous cells ; the cells of the medullary rays generally retain their living contents for a long time, but those of the pith live for a short time only. -- VASCULAR CYLINDER OR STELE ii; If we select an individual bundle in the internode of almost any dicotyledon and trace it upwards it will be found to pass out of the stele across the cortex and into the leaves, where it branches and forms the veins. Bundles of this kind common to both leaf and stem are termed common bundles, that part of each present in the stem being spoken of as the leaf-trace of the bundle. From each leaf one or several bundles may enter the stem, and on being followed downwards they are found to descend perpendicularly through one or more internodes, finally uniting with bundles which have entered the stem from older leaves lower down. The bundles in their descent all keep about the same distance from the centre, so that in a transverse section they appear arranged in a circle. Great variation exists in the manner and amount of branching and union of the bundles in different plants, but the arrangement is always such that the vascular bundles of the leaves, stems and roots form a continuous conducting system of tissues specially adapted to facilitate rapid and easy transmission of sap to all parts of the plant. In this type of stem each vascular bundle consists of three kinds of tissue, namely : (i) xylem or wood (n, i. Fig. 55); (2) phloem or bast (d) ; and (3) a thin-walled meristem tissue termed the cambium of the bundle {c). These tissues are arranged side by side in such manner that in a transverse section of the stem a radius drawn from the centre to the outside passes through all three ; the cambium lies between the wood and the bast, the wood being nearest to, and the bast farthest away from the pith. Bundles in which the wood and bast lie on the same radius are termed collateral bundles; when as in dicotyledons they also possess cambium they are said to be open. -- (a) Wood or xylem. The elements met with in the wood ; dast Fig. 55.-- I. Transverse section of a vascular bundle of a sunflower X stem (enlarged about 120 diameters). Enlargement of i^^ previous fig. 2. Longitudinal radial section through the same. / Medulla of stem ; n the wood ; d the bast ; c the cambium of the bundle ; a spiral vessel /"fibre ; pitted vessel ; s sieve tube ; t companion-cell ; b pericj'cle fibres; endodermis. : VASCULAR CYLINDER OR STELE 1 19 are usually (i) vessels or trachece, (2) tracheids, (3) fibres and fibrous cells, and (4) wood-parenchyma, all of which commonly have much thickened firm cell-walls consisting of lignocellulose. The proportion is not the same in all bundles and in some cases certain elements are missing altogether ; tracheas or tracheids, however, are constantly present in all wood. The vessels or trachete {a and 0) are not cells, but long con- tinuous open tubes, each formed from a row of superimposed cells, many of the transverse cell-walls of which have been absorbed or dissolved away. In some climbing plants the cavities of the vessels are 9 or 10 feet long : according to Adler's measurements, the vessels of oak wood average about 40 inches long, those of hazel and birch about 5 inches. Their walls always exhibit either annular, spiral, or reticulate thickening or pits. Those first formed in the bundle possess only annular or spiral thickenings, and constitute the protoxy- lem. At first all vessels contain protoplasm, but during their growth the living substance is used up in the thickening of the cell -walls when fully formed they are dead empty structures which serve for the conduction of water. Tracheids resemble vessels in the character of their cell-walls and in their function : they are, however, long, single, empty cells and not compound structures. The fibrous cells are long and pointed at both ends ; they possess living contents and their cell-walls are most frequently thickened and sometimes marked with small pits. Fibres (/) are similar thick-walled cells which have lost their protoplasmic contents and contain air or water only. The wood-parenchyma consists of somewhat elongated cells with square, blunt ends and living contents : the cell-walls are thickish and slightly pitted. In these cells starch is often stored. -- {b) Bast or phloem. The elements composing the bast or phloem are (i) sieve-tubes or bast-vessels {s) with their companion- 120 ANATOMY OF STEM, ROOT AND LEAF cells (f), and (2) a certain amount of thin-walled bast-parenchyma : their cell-walls consist of ordinary cellulose. The bast-vessels are long thin-walled cells arranged end to end. The transverse or end-walls which separate one vessel from another, aire not completely absorbed as in the vessels of the wood, but merely perforated by open pores through which the contents of adjoining vessels are in continuous open communication : these transverse perforated walls are called sieve-plates. When mature the bast-vessels contain a thin lining of cytoplasm but no nucleus : the rest of the cell-cavity is filled with an alkaline slimy substance, rich in proteids, and frequently containing starch-grains as well. The bast-vessels serve for the conduction of various complex organic substances, but more especially for those of a proteid character. The companion-cells are long narrow cells which lie alongside the sieve-tubes : they are filled with granular cytoplasm in which a nucleus is always present. Both the sieve-tube and its companion-cell arise from the same mother-cell. -- {c) Cambium. The cambium lies between the wood {c, Fig. 55) and the bast, and consists of a layer of thin-walled meristematic cells, each of which has the form of a long, narrow, rectangular prism with obliquely pointed ends. In young stems the cambium is confined within the vascular bundles, but in older ones a new and exactly similar meristematic tissue termed the interfascicular cambium arises in the medullary rays, and extends across the latter, joining the We cambium of one bundle with that of the next {ic, Fig. 54). thus have in the older stems a thin complete cylinder of dividingcells which in transverse section appears as a narrow zone, spoken of as the cambium-ring. The cambium-ring adds new elements to the wood and bast of the stem in a manner explained below; but in short-lived herbaceous dicotyledons this additional growth soon ceases, so '. VASCULAR CYLINDER OR STELE I2\ that its effect is not so noticeable in these as in perennial woody stems. -- Ex. 56. Cut across the young soft stems of the sunflower, Jerusalem arti- choke, groundsel, bean, potato, and any other common herbaceous plants. Examine the cut surfaces with a pocket lens, and observe the presence and arrangement of the vascular bundles and pith. -- Ex. B6. Place some young sunflower stems in a mixture of two-parts methy- lated spirit and one-part of water. Keep them in this mixture for further use. From a stem which has been in the mixture three or four days cut very thin transverse sections with a razor wetted with the mixture. Transfer the sections to a watch glass containing water ; after remaining in the water for a few minutes, take one out and mount it in a drop of water on a glass slide. Cover with cover-slip and examine with the lowest power of the microscope. Make drawings indicating the position and general character of the {a) epidermis, {d) cortex, {c) endodermis, (rf) vascular bundles, and (�) pith and mei'ullary ray tissue between the bundles. Examine with a high power, and make sketches of small portions of the various parts above-mentioned, paying especial attention to the wood, cambium and bast (compare Fig. 55). Try and see if the interfascicular cambium has been formed across the medullary rays. -- Ex. 57. Take a piece of sunflower stem about a quarter of an inch long, preserved as in preceding exercise, and cut longitudinal sections so as to pass through a vascular bundle. (In cutting longitudinal sections of stems, the razor should cut from one side of the stem to the other, not from end to end.) Examine first with a low and then with a high power : make sketches of the form of the cells met with in the epidermis, cortex, bast, cambium, wood and pith respectively. Try and determine which cells of the longitudinal section correspond with those seen in the transverse sections. -- Ex. 58. Make a careful study of the anatomy of a stem of groundsel, bean, and other common herbaceous dicotyledons. Always begin the examination of sections with the lowest power at disposal, namely, with the naked eye or a good pocket lens. After the general arrangement of the chief tissues is understood, then apply higher powers in succession. ; 122 ANATOMY OF STEM, ROOT AND LEAF B. The perennial woody stems of dicotyledons. -- (a) Division of the cambium-cells. In the earliest stages of the stems of shrubs and trees the arrangement and constitution of the tissues are essentially the same as in simple short-lived herbaceous stems. With an increase in age there is, however, a steady increase in thickness from year to year, and in transverse sections of such thickened stems the isolated small vascular bundles, so obvious when the stems are very young and soft, are no longer visible. The greatest part of the increased bulk of tissues in such stems as these, is brought about by divi- sion of the initial cells of the cambium-ring. Each initial cambium-cell (a. Fig. 56) divides in two by a wall parallel to the surface of the stem ; one of these two daughter-cells remains permanently capable of division while the other is either directly converted into a permanent cell, or divides once or twice, after -- Fig. 56. Transverse section through a small portion of the cambium-ring in a young black currant shoot. �: Cambium; a initial cell ; 7(/wood ; ^bast; m medullary ray. (Enlarged about 450 diameters.) which the cells produced become gradually changed into permanent elements. The change into a permanent cell or cells may happen to either of the two produced by division of the initial cell if the inner one is modified it is added to the wood i^v), if the outer one is altered it goes to increase the bast {U) Division of the cambium-cells, and the growth and development of the products continue from spring to autumn ; in winter, cell-division ceases. Since the cambium extends in the form of a continuous cyHnder within the stem, a new cylinder of wood is added every growing season to the outside of that already ANNUAL-RINGS : KNOTS 123 present, and a similar addition is made to the bast on its inside. The amount of wood produced by the cambium is always very much greater than the bast. Moreover, the bast tissue consists chiefly of thin-walled elements which become crushed into very thin sheets by the pressure of the expanding wood and the resistent bark, whereas the wood with its thick-walled cells and vessels suffers little in this manner ; in transverse sections of the ^T A-- -- Fig. 57. I. Piece of a stem of an ash tree. A, Portion three years old ; �, portion two years old. 2. Longitudinal and transverse sections of same. trunks and branches of trees and shrubs the cambium appears to the naked eye to produce wood only. -- (i) Annual rings : knots. If a tree is sawn across and the cut surface then smoothed with a chisel a number of ring-like zones are noticeable in the wood (Figs. 57 and 58); these are ;; 124 ANATOMY OF STEM, ROOT AND LEAF termed annual-rings and each represents the wood-tissue produced by the cambium during one active vegetative period. From the beginning of one vegetative period to the commencement of another is generally one year, so that in a two-year-old stem two rings are visible, in one three-year-old three rings are seen, and so on (Fig. 57). It is on account of certain differences between the wood made at the commencement of the growing season and that produced at the end that we are able to recognise these successive yearly additions to the wood as distinct bands, for if the elements produced by the cambium were of exactly similar character throughout its life, it would not be possible to determine the points at which the cambium had ceased or recommenced its growth. When the cambium commences growth in spring it gives rise to vessels and cells with thinner walls and wider cell-cavities than those which it manufactures in late summer and autumn in each annual ring (r, Fig. 64), therefore, two more or less distinct portions are visible, namely, (i) a layer of spring-wood {s) produced early in the growing season, and (ii) a layer of what is termed autumn-wood (a) produced in late summer and autumn. The spring-wood is generally of soft nature and pale colour ; in oak, elm, ash, and Spanish chestnut its vessels are so wide that they appear to the naked eye as a zone of pores. The autumn-wood is harder and generally of darker colour fewer vessels are present in it, and they are usually too small to be seen with the naked eye. The cambium of a stem is continuous with that of its branches, and in a longitudinal section (Fig. 58) the annual increment to the wood of the stem is seen to be continued in the branches, although in the latter the amount added per annum is smaller than in the stem, and consequently the annual rings of a branch are narrower than those of same age in the stem. It will be seen from the above Fig. that the basal portions ELEMENTS PRODUCED EV THE CAMBIUM I25 of a branch become buried by the wood added to the stem year by year : on cutting a longitudinal board as indicated at C, the buried part of the branch is cut almost transversely, and appears as an oval knot {k). -- (c) Elements produced by the cambium : medullary rays. As the cambium lies between the wood and bast, it is obvious that 'Cas, primary first-formed wood and bast of the vascular bundles Fig. S%.--A, Stem of a tree six years old with branch b ; B, longitudinal section of the same, showing all annu.il rings of stems except the first continued in branch; D, longi- A tudinal board cut from ; k knot (transverse section of branch i>). must be gradually pushed further apart by the secondary wood and bast produced by the cambium, so that in old stems the primary wood is found surrounding the pith in the centre, while the primary bast is met with near the outside (C, Fig. 60). The elements forming the secondary wood are similar to those 126 ANATOMY OF STEM, ROOT AND LEAF of the primary wood, namely, tracheae or vessels, tracheids, fibres, fibrous cells and wood-parenchyma; the vessels and tracheids, however, are never spirally or annularly thickened, but usually marked with bordered pits and reticulate thickenings. All these elements may be present or only a few ; for example, the wood of the yew consists of tracheids only, that of the bulk of coniferous trees of tracheids and wood-parenchyma, while the wood of most dicotyledons contains all the above-mentioned elements. The elements of the secondary bast are similar to those of the primary bast, namely, sieve-tubes with their companion-cells and parenchyma; bast-fibres and living fibrous cells are also present in some cases. After functioning for a short time as conductors of food, the sieve-tubes, companion-cells and most of the bast-parenchyma become empty, and in the older parts are compressed into an irregular mass in which no cell cavities are visible. When firm thick-walled bast-fibres are abundant, as in lime and other trees, the bast in transverse sections appears in the form of thin, ring-like bands. Besides the production of wood and bast, certain cells of the cambium-ring become changed into medullary ray cells {m, Fig. 56); the primary medullary rays existing between the first-formed vascular bundles of the unthickened stem are continued by the interfascicular cambium when thickening begins and therefore always extend right through from the pith to beyond the bast. Totally new secondary medullary rays are subsequently started by certain cells of the cambium ring at successive irregular intervals during the growth in thickness. These new medullary rays extend from the annual rings of wood in which they first appear to the corresponding bast rings on the opposite side of the cambium ; they are therefore of variable length. The medullary rays are of variable width even in the same stem. Sometimes they are only one cell thick and in trans- HEART-WOOD AND SPLINT-WOOD 127 verse sections are scarcely visible to the naked eye, while in oak, beech and other kinds of timber, many of them are several cells thick, and in transverse sections appear as distinct lightcoloured radial bands (in, Fig. 64). In true radial longitudinal sections, when seen at all, they appear as transverse bands of variable vertical diameter running from the pith outwards (Fig. 62), the primary rays have the greatest vertical breadth. In longitudinal sections cut obliquely to the radius of the stem small portions only are visible as bran-like spots. The cells of the medullary rays are brick-shaped, generally with thick pitted walls and living contents, which they often retain for a long time. They conduct various food-products manufactured in the leaves, and in winter starch and various food-substances are stored in them for use in the following season. Air circulates to all parts of the wood and bast in the intercellular spaces be- tween the medullary ray cells. -- {d) Heart-wood and splint- wood. In the old stems of oak, walnut, larch, yew and other trees, the wood of the annual rings in the centre of the tree is heavier, harder, darker in colour, and drier than that of the younger rings near the cambium : this dark wood is known as heart-wood or duramen, while the lightcoloured softer wood surrounding it is termed splint-wood, sapwood or alburnum. The width of the splint-wood or the number of annual rings over which it extends is not the same in all trees, nor is it always the same in the same species of the same age. The splint-wood is the part which conducts the ' sap ' and many of its parenchymatous cells are still living : starch, sugar and other compounds readily attacked by fungi are generally stored in it, and from its liability to rot it is valueless as timber. The heart-wood acts as a strong support for the rest of the tree : its