Plant tissue

“ The bark or cellular tissue of these leaves is penetrated by the fibres of the wood in the shape of reins, veinlets and capillaries, which communicate directly with the fibres of the stem and roots, and thus act as conduits of the sap from one extremity of the plant to the other. In this manner the sap brought from all the other parts of the plant is conducted to all parts of the leaf by these veins, veinlets and capillaries, to be thoroughly spread out and aerated in the leaves. ”

“ The primary state of all the tissues of the plant is the condition of simple cells, each of which is, in a certain sense, an independent body, having its own life-work and history in the complicated mass of which the tree is composed. All extension of the tree in any direction is made through the medium of cell-growth and cell-modifications, and, wherever there is cellular tissue in a state of vitality, there may be cell-multiplication whenever material for growth, i. e., sap in different stages, is brought into contact with such tissue. ”

“ It is the persistence of permanently juvenile tissue in certain regions of a tree, together with the remarkable power of repairing injuries and shedding effete parts, that constitute some of the most striking contrasts between the higher animals and plants. The embryo oak in the earlier stages of development consists entirely of actively growing cells; by degrees differentiation of the embryonic tissues results in the « 10 » localisation of regions of cell-production at the tips of the elongating stem and root. ”

“ In proportion as the inner structure of plants was better known and the different kinds of tissue better distinguished, it became possible to bring the functions of organs, as made known by experiment, into connection with their microscopic structure; phytotomy dissected the living machine into its component parts, and could then leave it to physiology to discover from the structure and contents of the tissues, how far they were adapted to perform definite functions. Obviously this only became possible when the phenomena of vegetation had been previously studied in the living plant. ”

“ It exists in many forms with widely-differing physical properties, but with very little variation in chemical composition—so little, that in many chemical treatises cellular tissue, cellulose, lignin, and woody fiber are treated as chemically synonymous. Thus, Miller says: "Cellular tissue forms the groundwork of every plant, and when obtained in a pure state its composition is the same, whatever may have been the nature of the plants which furnished it, though it may vary greatly in appearance and physical characters ”

“ In the blade of a typical leaf of a vascular plant—essentially a thin plate of assimilating tissue—the vascular system takes the Stelar Tissue of Leaf and Root. form of a number of separate, usually branching and anastomosing strands. These, with their associated stereom, form a kind of framework which is of great importance in supporting the mesophyll; but also, and chiefly, they provide a number of channels, penetrating every part of the leaf, along which water and dissolved salts are conveyed to, and elaborated food-substances from, the mesophyll cells. ”

“ Phyllody in accessory organs.—In addition to the ordinary organs of the plant, what are termed the accessory organs, such as hairs, spines, &c., sometimes become foliaceous. It is not to be wondered at that spines, when they represent the framework of a leaf, become sometimes clothed with cellular tissue, and thus become indeed true leaves. This happens occasionally in Berberis; a similar thing occurs in the stipules of some Leguminosæ ”

“ Your plants, you know, are composed chiefly of water, which fills innumerable little cells formed by the vegetable tissue. If the water in the cells is chilled beyond a certain point, if it becomes solid ice, it expands and breaks down the tissue of the cells, and the structure of the plant is destroyed. If the frost can be gradually withdrawn so as to leave the cells substantially intact, they can eventually resume their functions, and the plant receive no very great injury. ”

“ Even the most delicate tissues, such as cambium and phloem, the endosperm of seeds, or the formative tissue of the growing-point, are frequently preserved cell for cell, both in calcareous and silicious material. As a rule, the petrified remains, all-important for the revelation of structure, are fragmentary, and give little idea of the habit or external characters of the plants from which they were derived. Hence they must be brought into relation with the specimens preserved as casts or impressions, in order to gain a better conception of the plant as a whole. ”

“ You will remember that the surface of leaves is continually giving off water from its many pores. When a plant has a number of big leaves this transpiring area is large, while when it has no leaves at all, but a thick, green stem instead, then the amount of surface from which water vapour is being given off is very much reduced, even though there may be about an equal quantity of actual tissue in the two plants. ”

“ In age the pits become open passages, facilitating the passage of sap and air.
409. The proper cellular tissue, or parenchyma, is the ground-work of root, stem, and leaves; this is traversed, chiefly lengthwise, by the strengthening and conducting tissue, wood-cells and duct-cells, in the form of bundles or threads, which, in the stems and stalks of herbs are fewer and comparatively scattered, but in shrubs and trees so numerous and crowded that in the stems and all permanent parts they make a solid mass of wood. ”

“ The cells must turn a broad face to the outside; this they do by becoming arranged in branching rows, or in leaf-shaped flattened organs. That they may suck up water and salts from the soil, the cells are arranged as a highly branched system of roots, covered with delicate hairs, and penetrating the soil in every direction. To inhale the carbonic acid from the air, and to be subjected to the influence of sunlight, the aerial part of the plant stretches out its branches towards the light, and becomes folded into the flat leaves, the structure of which reveals a suitability for assimilation. ”

“ A microscopical examination of the tissues of the plant shows them to be comparatively simple, this being especially the case with the fibro-vascular system.
The epidermis of the leaf is readily removed, and examination shows it to be made up of oblong cells with large breathing pores in rows. The breathing pores are much larger than any we have yet seen, and are of the type common to most angiosperms. ”

“ They breathe when the leaf mouths take from the air the oxygen, and give back to it carbonic-acid gas.
The veins and veinlets, of which you see so many running through a leaf, act in something the same way as the water pipes of a city; for through these veins the watery food, the earth broth, is carried to the different cells.
When one knows all that we know even now about a plant, one looks at a tree covered with leaves with a good deal of admiration. ”

“ Sometimes this new cell is needed to help do the work of a root, and so it begins to do this work, and becomes part of the root; or else it is needed to do stem work, and goes to make up the stem, or leaf work, and is turned over into the leaf.
A healthy cell is born with the power to do whatever is most needed.
HOW A PLANT’S FOOD IS COOKED Some time ago we learned that the little root hairs, by means of their acid, are able to make a sort of broth from the earthy materials which they could not swallow in a solid state. ”

“ In the first place, the enormous holdfast, ensured by the millions of points of adherence, enables the plant to withstand even powerful lever actions from above, and provides fixed points against which the root-tips can work as they drive deeper into the soil. In the second place, the intimate contact of the root-hairs and particles of soil, ensures that the films of water held by surface-action on the soil-particles and root-hairs shall be in continuity with the water saturating the cell-walls of the latter, and therefore with the protoplasm and cell-sap in their interior. ”

“ We do not aspire to a treatise on botany, but stop to consider only certain facts which are essential to this argument; a consideration of the higher plants will serve our purpose. Certain phytons are modified to become roots, which are the organs devoted to the absorption from the earth of the materials which are to be woven into the plant; other phytons become the stem for support; others the branches for expansion; others the leaves for respiration; others pistils and stamens for reproduction, while others become floral envelopes for their protection. ”

“ But the whole plant, often many feet in length, consists only of single gigantic cells. A tough skin incloses its juices, which flow in a continuous stream through the stem, leaves, and roots of the curious growth, here taking up through the skin and assimilating mineral substances, there producing and transforming organic matter, and at the same time advancing the growth and increase of the whole. Not less peculiar than the inner structure and appearance of the stem is the form of the leaves of water plants. Their service to the organism is the same with that of the leaves of other plants. ”

“ The vegetable cell, on the contrary, is surrounded by a membrane of cellulose, which condemns it to immobility. And, from the bottom to the top of the vegetable kingdom, there are the same habits growing more and more sedentary, the plant having no need to move, and finding around it, in the air and water and soil in which it is placed, the mineral elements it can appropriate directly. It is true that phenomena of movement are seen in plants. ”

“ When alive they are filled with cell sap and protoplasm.
Trace the woody strands through the nodes. Do they ascend vertically? Do they curve toward the rind at certain places? Compare their course with the strands shown in Fig. 70. The woody strands consist chiefly of tough fibrous cells that give rigidity [61] and strength to the plant, and of long tubular interrupted canals that serve to convey sap upward from the root and to convey food downward from the leaves to the stem and the roots. ”

“ Receiving the water from the soil with such materials as it brings thence in solution, and absorbing carbonic dioxide and ammonia from the air, the living protoplasm of the leaf-cells has the power of chemically changing all these substances, and of producing from them those complicated and otherwise inimitable organic compounds of which the tissues of the plant are built up. ”

“ In the higher grades of plants it will be evident that the several tissues that compose their bodies are not found in the root, stem, and leaf without definite order and purpose, but that they are grouped into systems for the performance of different kinds of work. In all flowering plants at least three different systems may be clearly distinguished. These are the epidermal or boundary tissue system, the fundamental or ground tissue system, and the fibro-vascular or conducting system. All three systems of tissue originate from meristem cells, located at the growing point of the stem and root. ”

“ When we reflect that the constant removal of water in the leaf, both as such, and as the only carrier of food supplies from the roots, depends in such large measure upon the functioning of these guard cells, then we come to some realization of their importance to the plant. ”

“ At the base of the leaflets of a compound leaf, small stipules (stipels) are occasionally produced.
Variations in the structure and forms of leaves and leafstalks are produced by the increased development of cellular tissue, by the abortion or degeneration of parts, by the multiplication or repetition of parts and by adhesion. When cellular tissue is developed to a great extent, leaves become succulent and occasionally Modifications. ”

“ You know that they are stowing away their food in warmer, safer places than can be supplied by the delicate leaves. And when all the food has been drawn out of the leaves, and packed away in the right spots, then the plant finishes a piece of work it began some time before. This piece of work is the building-up of a row of little cells just where the leafstalk joins the stem or branch. When this row is complete, it acts almost like a knife, loosening the stalk from the stem. ”

“ All such soft cellular tissue, like this of leaves, that of pith, and of the green bark, is called Parenchyma, while fibrous and woody parts are composed of Prosenchyma, that is, of peculiarly transformed
407. Strengthening Cells. Common cellular tissue, which makes up the whole structure of all very young plants, and the whole of Mosses and other vegetables of the lowest grade, even when full grown, is too tender or too brittle to give needful strength and toughness for plants which are to rise to any considerable height and support themselves. ”

“ It passes through the stomata by diffusion, dissolves in the water, saturating the cell-walls, and so enters the cells. The organs by which the food is made are the minute, green bodies called chloroplasts or chlorophyll bodies, which give the plant its color. They are composed of protoplasm, which holds a green dye, chlorophyll, and are imbedded in the colorless protoplasm of those cells which lie near the surface of a plant. ”

“ The mechanical tissue of the stem is arranged in the form of solid or hollow columns in order to sustain the enormous weight of the branches. In roots the mechanical tissue is combined in ropelike strands, thereby effectively resisting pulling stresses. The epidermis of leaves subjected to the tearing force of the wind has epidermal cells with greatly thickened walls, particularly at the margin of the leaf. The epidermal cells of most seeds have very thick and lignified cell walls, which effectively resist crushing forces. ”