Stroma

“ So we find that if roots are uncovered, they will turn away from the light and burrow into the earth again. They avoid light just as a worm would do.
Roots are of course intended to absorb or suck in water. If there is a drain in the soil or a place where water collects, the roots will grow towards that place. Very often they form a dense spongy mass of fibres which may almost choke the drain. Along a riverside one can often find great fibrous masses of tree roots near the water. ”

“ He observes that water-plants, both those that grow submerged and those which appear on the surface, are subject to a variety of modifications in the forms of their stems, leaves, and other organs, according as the water in which they grow is in more or less lively motion. Plants which grow in a stream, and are rooted in the ground, seem to receive a kind of pull from the moving force of the water, which is proportioned to the speed of the current. ”

“ Let us imagine, in the first instance, such trees as those represented in the figures, growing thickly together over vast swampy flats, with quantities of undergrowth of ferns and other plants beneath their shade, and accumulating from age to age in a moist soil and climate a vast thickness of vegetable mould and trunks of trees, and spores and spore-cases, and we have the conditions necessary for the growth of coal. ”

“ When we touch the leaf, we give it another motion more violent than the first: this overcomes the first: the vibration is stopped by the rude shock: and the leaves close, and their foot stalks fall, because that vibrating motion is destroyed, which kept them elevated and expanded....That the power of motion in the sensitive plant depends upon the effect of light on the expanded surface of the leaves, is certain; for till they are expanded, they have no such power. ”

“ As the kernel swells with heat and moisture, its starches and albumens get oxidised and produce the motions and rearrangements of particles that result in the growth of a new plant. First a little head rises towards the sunlight and a little root pushes downward towards the moist soil beneath. The business of the root is to collect water for the circulating medium—the sap or blood of the plant—as well as a few mineral matters required for its stem and cells ”

“ For a leaf is only a flat piece of radiation; but the tree throws its branches on all sides, and even in every profile view of it, which presents a radiation more or less correspondent to that of its leaves, it is more beautiful, because varied by the freedom of the separate branches. ”

“ The stem, the branch, and the root, are alike constituted of an outer coating which affords a mechanical support, and once formed takes no further share in the economy of the plant, excepting that of assisting to convey fluid from the roots to the branches and leaves, an action more of a capillary nature than vital. The nourishment of the plant is brought about by other material structures, as the pith, the cortex, the cambium, and so forth, all of which greatly assist in the formative process. ”

“ The water, or liquid, when it mounts a stem or tree trunk, takes a path that leads through the new-made cells. Each young cell wall is made of such delicate material that it allows the water, or broth, to filter through it, just as it would pass through a piece of thin cloth. And so it makes its way from cell to cell, along the stem, more slowly than if it were passing through a hollow tube, but almost as surely. It is true, the earth broth does not reach the leaves above without having given up something to the little cells along the road. ”

“ The influence of light is chiefly upon the leaves, that of heat upon the earth and root, which becomes more fully developed. When a seed is placed in the earth, moisture is absorbed by it from the earth, and a development of the cells of the cellular tissue of which the seed is composed takes place, and from the peculiar nature of these cells they [Pg 122] divide into two sets, one of which passes upwards towards the light, the other passes downwards into the earth ”

“ This separation of the tissues and systems, with the endeavour upon the part of each to become individually perfect and isolated from the other, is effected by the air and the light. As root imbibes the mucous or slimy water, and sustains the chemical process in the plant, so does the stalk set the water in motion, since it exposes it to the air and light, whereby the chemical earth-process becomes separated into different saps and elemental bodies. ”

“ A plant has to pass through two phases, one of vegetation, the other of propagation, both of which are conditioned upon the supply of food. In the simplest instances these phases are exhibited in one and the same cell, which, absorbing the food, elaborates it for its own growth, and gathers so much force that two new individuals are produced by division while the mother plant ceases, by this fact, to be. ”

“ The exact mechanism of the movement in the stem itself is too complex for discussion here; but I may sum up the matter by saying that the disappearance of the water at the surfaces of the leaves starts a series of flows in directions of least resistance towards the mesophyll, and as long as the evaporation goes on more water flows into the cells, to replace that lost, from the vessels of the stem, when the water-columns are supported and moved partly by capillarity and by the air-bubbles in the cavities, and partly by a peculiar co-operation of the living cells of the medullary rays. ”

“ When cells are changed into new forms, or "differentiated," as it is called, they become permanent tissues. These permanent tissues of the tree trunk constitute the various parts which we have noticed, viz., the rind, the pith and the wood.
The essentially living part of the tree, it should be remembered, is the protoplasm: where there is protoplasm, there is life and growth. In the stems of the conifers and broad-leaved trees—sometimes together called exogens—this protoplasm is to be found in the buds and in the cambium sheath, and these are the growing parts of the tree. ”

“ By cutting sections of these nodules and grinding them to a transparent thinness, the most delicate tissues of Carboniferous plants are rendered accessible to investigation under the high power of a microscope. As our attention is absorbed by the examination of the details of cell-structure it is easy to forget that the section has not been cut from a living plant, but from the twig of a tree which grew in the forests of the Coal age. ”

“ Even those plants which, like the water marestail, can stand up by themselves some way out of the water, yet have softer stems than most land-plants, and far fewer well-developed “water-pipe” cells, because they are so surrounded by water that they can get it easily. Both these plants and the water-lilies, as well as many others, store air rather than water in their stems, and often the spaces in the meshes of the stem-tissue are filled with air, which acts both as an air reservoir and a buoy to float the leaves. ”

“ Now, the saps and juices of plants are thicker than water, so, directly the water enters the cells at the root it oozes up into the cells above, and mixes with the sap. Then the matter in those cells becomes thinner than in the cells above, so it too oozes up, and in this way cell by cell the water is pumped up into the leaves.
When it gets there it finds our old friends the sun-beams hard at work. If you have ever tried to grow a plant in a cellar, you will know that in the dark its leaves remain white and sickly. ”

“ The living contents of young and active cells are mainly protoplasm with water or watery sap which this has imbibed. Old and effete cells are often empty of solid matter, containing only water with whatever may be dissolved in it, or air, according to the time and circumstances. All the various products which plants in general elaborate, or which particular plants specially elaborate, out of the common food which they derive from the soil and the air, are contained in the cells, and in the cells they are produced. ”

“ Each cell may be considered a distinct individual plant, since it is perfectly independent of others with which it may be aggregated, and performs for and by itself all the functions of growth and reproduction, having a containing membrane which absorbs liquids and gases from the surrounding matrix or elements, a contained fluid of peculiar character, formed out of these materials, and a number of excessively minute granules, equivalent to spores, or, as some would say, to cellular buds, which are to become the genus of new plants. ”

“ 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. ”

“ If a leaf has its surface moistened and particles of water separate from each other as in a mist be brought near the moistened surface of a leaf, each particle will be attracted more by that plain surface of water on the leaf than it can be by the surrounding particles of the mist, because globules only attract each other in one point, whereas a plain attracts a globule by a greater extent of its surface. ”

“ In the sections of the sea-weeds, therefore, even as made for the scientific elucidation of their structure, we may expect to find, as we undoubtedly shall do, many hints and lessons.
The true form of the cell is perhaps the globe, but it is more commonly presented to us as the cylinder, the conditions and outlines of which are varied almost ad infinitum, as by the various effects of growth and pressure the cells are forced into hexagons, pentagons, and other mathematical shapes, or their lines of junction are disposed in undulating tracery of the most elegant and intricate patterns. ”

“ The organic action of the tree, as thus far described, tends to the desiccation of air and earth; but when we consider what volumes of water are daily absorbed by a large tree, and how small a proportion of the weight of this fluid consists of matter which, at the period when the flow of sap is freest, enters into new combinations, and becomes a part of the solid framework of the vegetable, or a component of its deciduous products, it becomes evident that the superfluous moisture must somehow be carried back again almost as rapidly as it flows into the tree. ”

“ If, as already pointed out, the transverse growth of an axis arises when the axis comes to be a channel of circulation between all the roots at one of its extremities and all the leaves at the other; and if this lateral bulging must increase as fast as the quantity of foliage to be brought in communication with the roots increases—especially if such foliage has at the same time to be raised high above the earth’s surface ”

“ The relatively rigid nature of the plant cell-wall, and the attenuated inorganic food-supply of plants, make possible and necessary a form of growth in which the greatest surface is exposed to the exterior, and thus the plant body is composed of flattened laminae and elongated branching growths. ”

“ If the protoplasm contains visible granules, as it usually does, within a clear external layer, we see that these stream constantly forwards along the central axis of each process as it forms, and backwards just within the clear layer all round, like a fountain playing in a bell-jar. ”

“ The lecturer first described the infinite variations in life reactions in plants. The same external stimulus, he said apparently produces one effect in one plant; and precisely opposite in another. Some leaves move towards light; others are repelled by it. The root bends towards the centre of the earth, the shoot rises above away from it. Numerous other "tropic" movements are caused by [Pg_227] contact, by electricity, by moisture and by invisible radiations. ”

“ However, to perform this attraction and repulsion, one condition is requisite which is equally necessary in the life of plants, and that is the circulation of the fluids. This circulation we have seen performed in the plant through the polarity between the root and the flower, and therefore in a linear direction from one end to the other. ”

“ It is a curious fact with regard to bell-heather growing in higher latitudes, that the plants possess a peculiar root structure as a protection against droughts. In most of them the sustentation of life depends upon the formation of a number of long thin filaments on their roots resembling root-hairs, which penetrate the root, forming nodular masses within it. ”

“ The fibro-vascular bundles of the stem of the seedling plant show a structure quite similar to that of the leaf, but very soon a difference is manifested. Between the two parts of the bundle the cells continue to divide and add constantly to the size of the bundle, and at the same time the bundles become connected by a line of similar growing cells, so that very early we find a ring of growing cells extending completely around the stem. ”

“ Although it is easy to perceive, why each separate air-cell, or each fibre in pumice-stone, [47] should be drawn out in the direction of the moving mass; it is by no means at first obvious why such air-cells and fibres should be arranged by the movement, in the same planes, in laminæ absolutely straight and parallel to each other, and often of extreme tenuity; and still less obvious is it, why such layers should come to be of slightly different composition and of different textures. ”