46 PHYSIOLOGY [VEGETABLE. such as sulphur dioxide, sulphuretted hydrogen, and hydro chloric acid, which are occasionally present in the air as impurities, are absorbed by the leaves, as is shown by the pernicious effects which they produce. Circulation. Osmotic It is obviously necessary, in multicellular plants in which circula- certain cells only are in a position to absorb food-materials IOU- from without, that these food-materials should be conveyed from the absorbent cells to the remainder of the plant. In no plant is there any organ comparable to the heart of animals by means of which a distribution throughout the tissues of absorbed food-materials is effected. The distribu tion is accomplished by purely physical means, principally by osmosis. When the cell-sap of a cell becomes charged, by absorption from without or from neighbouring cells, with any substance, diffusion-currents are at once set up between this cell and any adjacent cells the cell-sap of which may contain the substance in question in smaller proportion, and these currents will persist until osmotic equilibrium, as far as this substance is concerned, is estab lished. The diffusion-currents do not flow in any definite direction, but their course is determined simply by inequali ties in the chemical composition of the cell-sap of the cells in different parts of the plant. Since in subaerial plants the roots are as a rule the only organs which absorb sub stances from the soil, and since the cell-sap of their cells is therefore relatively rich in absorbed food-materials, the general direction of the diffusion-currents is from the roots upwards into the stem and leaves. In cellular plants that is, in plants which possess no vascular tissue the distribution of absorbed food-materials is effected solely by osmosis. Many of these plants are small, so that the distribution is effected from cell to cell with sufficient rapidity by this means. Those of them that are large have a very considerable absorbent surface, many of them being aquatic in habit, so that the absorbed substances have no great distance to travel. In vascular plants, more particularly in those which are subaerial in habit, the distribution of the water, holding substances in solution, which is absorbed by the roots is effected to a considerable extent by means of the vascular system. The forces by which the flow of liquid through the vascular tissue is maintained are the following. The first is the Root- root -pressure. It is a matter of common observation pressure, that, when the stems of vascular plants are cut across, particularly in the spring, an escape of water takes place from the surface of that portion of the stem which still remains connected with the root, an escape which may persist for some considerable time. It has been ascertained that this outflow of water takes place under considerable pressure ; for instance, Hales observed, in the case of a Vine, that the pressure was sufficiently great to support a column of mercury 32J inches in height. But the root- pressure not only manifests itself by causing a flow of water from the cut surfaces of stems, it also causes in many plants the exudation of drops of water at the free surface. Drops may commonly be seen on the surface of certain Fungi (Piloholus crystal/inns, Penicillium ylaucum, Meruliits lacrimans), which are exuded in consequence of the hydrostatic pressure set up in the plant by the active absorption effected by the organs (rhizoids) which here perform the functions of roots. Again, drops are frequently to be found on the margins and at the apices of the leaves, especially the younger ones, of many plants, such as Grasses, Aroids, Alchemillas, Saxifrages, &c. That the formation of these drops depends upon the forcing of water upwards through the vessels by the root-pressure is proved by the fact that, if the stem be cut off from the root and then placed with its cut end in water, no more drops will appear on the leaves. The water thus forced into the vascular system is not pure water, but a watery solution of various substances, principally salts absorbed by the roots. It is therefore obvious that the root-pressure assists in the dis tribution of these substances throughout the plant. In order to understand how the root-pressure is set up Structu; it will be necessary to give a brief description of the of ro t. general structure of the root. It consists of a central tibro-vascular cylinder which is surrounded by several layers of parenchymatous cells, the most external of these layers being in contact with the epidermal layer, certain cells of which are developed into root-hairs. Water is absorbed by the root-hairs and passes from them by osmosis into the subjacent parenchymatous cells. It is obvious, however, that osmosis cannot take place between the cells of the innermost layer and the vessels, for the conditions of osmosis are not fulfilled, inasmuch as the vessels at first contain no liquid. The passage of water from the cells into the vessels can only take place by filtration. For this a certain pressure is necessary, and this pressure is set up by the absorbent activity of the root -hairs and of the parenchymatous cells. The system of cells absorbs large quantities of water, more indeed than the cells can contain, so that at length the resistance of the cell-walls is overcome at what is presumably the weakest point, and water filters into the cavities of the vessels of the wood. There it collects, and it may, under certain circumstances, fill the whole vascular system ; then, since absorption is still going on at the surface of the roots, sufficient pressure is set up to cause that exudation of drops on the leaves to which allusion has been made, and, if the stem be cut across, to cause " bleeding " at the cut surface. From the foregoing account it is apparent that the root-pressure is the expres sion of the absorbent activity of the root-hairs. But the vessels of the wood do not always contain water. Tran- Hales observed that, whereas a Vine will bleed freely if its s P iratil stem be cut across in the month of April, no bleeding is observed if it be cut in July. And yet it cannot be doubted that the plant is absorbing water by its roots more actively in July than in April. The explanation of these facts is that, although in July the plant is absorbing water ac tively by its roots, yet it is losing so much in the form of vapour from its leaves that water does not accumulate in the cavities of the vessels. This loss of water in the form of vapour from the general surface of the plant exposed to the air is termed " transpiration." The parts of the plant which are more especially concerned in transpiration are the leaves. By their structure they are peculiarly adapted for this purpose. The tissue of a leaf is penetrated in all directions by intercellular spaces, which communicate directly with the external air by means of the stomata in the epidermis. In this way a very large surface of moist and thin cell-wall is brought into contact with the air, a condition most favourable to evaporation. Some idea of the activity of transpiration in a plant is afforded by the following determinations made by Hales. In the case of a Sunflower with a leaf-surface of 5616 square inches the amount of water transpired during twelve hours of daylight was 30 fluid oz. (a pint and a half) ; in the case of a Cabbage with 2736 square inches of leaf -surface the amount of water transpired in the same time was 2f> fluid oz. The activity of transpiration is very much affected by Li^ht external conditions, the moister the air, the smaller will all(l be the transpiration ; and conversely, the drier the air and t the higher the temperature, the greater will be the amount of water transpired. Light, too, has a remarkable influence : it has been ascertained by a great number of observers that transpiration is more active in light than in darkness. It seems probable that this is to be attributed largely to
the influence of light upon the stomata. Each stoma is