VEGETABLE.] PHYSIOLOGY 47 fn- Bpition cmnt. D.nnel ) ran- i] ution usually bounded by two cells, termed "guard-cells," which are capable of so altering their form as to close or to open the aperture between them. The form of the guard-cells is dependent upon the amount of water which they contain. When they hold comparatively little water, and are flaccid, their adjacent free surfaces are straight and in contact with each other ; the stoma is then closed. When, how ever, they contain so much water that their cell-walls are under considerable pressure from within in a word, when the guard-cells are turgid they curve so that their ad jacent free surfaces are no longer in contact, but a space is left between them ; the stoma is then open. It appears that the guard-cells become turgid under the influence of light; and it is probably to this open condition of the stomata that the greater transpiration of leaves when exposed to light is to be ascribed. It is obvious that the effect of transpiration upon the dis- tribution of water through the plant is very great. It sets up a rapid current, known as the "transpiration-current," which travels from the roots upwards towards the leaves. Sachs has made some observations as to its rate by means of the lithium-method, which consists in supplying the root of a plant with a solution of a salt of lithium, and determining by means of the spectroscope the length of stem in which lithium could be detected after the lapse of a given time. He estimates the rate per hour to be in Nicotiana Tabacum 118 centimetres (46 "458 inches), in Helianthus annum 63 (24 793 inches), and in Vitis vini- fera 98 (38 583 inches). But the water of the transpira tion-current holds salts and other substances in solution. It is clear, therefore, that transpiration promotes the dis tribution not only of water but also of the substances which the water holds in solution. Sachs s experiments go to prove that salts travel in solution in the current ; hence it affords a ready means of transport of substances from the roots, where they are absorbed, to the leaves where (as will be shown below) the food undergoes certain changes which fit it for the nutrition of the plant. There is yet another important point to be noted with regard to the physiological significance of transpiration. It has been mentioned that the roots absorb from the soil only very dilute solutions of salts and other substances, so that for any given quantity of a salt absorbed an excessive quantity of water has to be absorbed likewise. It is obvious that the absorption of salts from the soil by the roots can only go on provided that the plant is able to get rid of the excess of absorbed water, and this is effected chiefly by transpiration, though, as mentioned above, an actual ex cretion of water in the form of drops not unfrequently takes place. It has been conclusively proved that the channel along which the transpiration-current travels is the fibre-vascular tissue, and that it is the xylem or woody portion of a fibro-vascular bundle which is the conducting tissue. In the case of plants like Conifers and Dicotyledons, in which there is a formation of secondary xylem or wood from a cambium-layer, it is the younger wood, the alburnum, along which the transpiration-current passes. The older wood, the duramen, it is true, usually contains water, but it does not serve as a conducting channel, only as a reservoir. The question now arises as to the mode in which the transpiration-current travels through the wood. Since the vessels contain no water in their cavities at the time when transpiration is most active, it is clear that it is not in the cavities of the vessels that the water of the current travels. Sachs is of opinion that it moves in the substance of the lignified cell-walls. Others, amongst whom Hartig may be especially named, consider that it travels from the cavity of one wood-cell to that of the next by filtration under pressure. The mechanism of conduction would, in the latter case, be this : the conducting cells contain air and water ; when water is withdrawn from one of them the contained air becomes rarefied, and the water in that cell is then subject to a lower pressure than that in neigh bouring cells ; as a consequence water is forced into the former cell through the thin membranes of the pits in its walls until equilibrium is re-established. Inasmuch, then, as the air in the conducting wood -cells in the leaves is constantly undergoing rarefaction in consequence of tran spiration, a current is set up towards- the leaves from the stem and the root. There can be no doubt, however, that, as Hales first Negative pointed out, transpiration has the effect of diminishing the pressure, pressure of the gases contained in the cells and vessels. Von Hohnel has found that, if the stem of a transpir ing plant be cut through under mercury, the mercury will at once rise to a height of several centimetres in the vessels, the greatest height being reached in the younger vessels. This rise can only be accounted for by ascribing it to the difference between the atmospheric pressure and the pres sure of the gases in the vessels, the lower pressure of the latter being due to the removal of water by transpiration, which necessarily involves an expansion, and therefore also a diminished pressure of the gases. The effect of this so- called " negative pressure " is to set up diffusion-currents of gases from the surrounding tissues into the cells and vessels of the fibro-vascular bundles. It must not be assumed, however, that the vessels are the principal channels in which gases circulate throughout the plant. They circulate principally in the intercellular spaces which communicate with the external air by means of the stomata. Stating the foregoing facts in the most general terms, Recapit- it appears that in a plant the food -materials travel by ulatiou - osmosis from the absorbent organs to the organs in which the processes of constructive metabolism are carried on in one of the higher plants, for instance, from the roots to the leaves and that the distribution of the food-materials is assisted and accelerated by root -pressure and by tran spiration, the fullest expression of this being the transpira tion-current in terrestrial vascular plants. And just as there is a current of food -materials tending towards the organs in which the processes of constructive metabolism are carried on, so also there is a current of the organic nutrient substances formed in these organs travelling from them to the other parts of the plant. The final cause of the current is the same in both cases. A given salt, for instance, which has been absorbed by the root travels towards the leaves because it is in some way undergoing chemical alteration in those organs ; similarly, a given organic substance formed in the leaves travels from them towards any part of the plant in which that substance is being chemically altered, or, to use a somewhat different expression, is being consumed. The cause of the diffusion in either case is the disturbance of osmotic equilibrium by the chemical alteration of the substance, and the result is a current of the substance from those parts which are relatively rich in it to those which are relatively poor. Distribution of Organic Nutrient Substances. In vas- Distribu- cular plants the distribution of the organic nutrient sub- tion f stances is, like the conduction of substances absorbed by ^jent the roots, assisted by the vascular tissue ; but, whereas it su ^_ is the wood which is the conducting tissue in the latter stances, case, in the former it is the bast or phloem, and more especially the bast-vessels or sieve-tubes. These vessels consist of elongated cells placed end to end, the septa be tween the adjacent cells being perforated so as to admit of a direct continuity between their protoplasmic contents. The importance of the wood and of the bast respectively as conducting tissues is well illustrated by the " ringing "
experiments which have been repeatedly made on plants,