CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 699 



distance into the woody tissue of tlie cut stem or branch. This is 

 due to the fact that, in consequence of the withdrawal of water 

 from them, the gases in the vessels are at a lower pressure than 

 that of the atmosphere. This is termed the negative pressure in 

 the vascular tissue. 



These various points can be readily observed in low -growing 

 plants, such as the cabbage. On a hot summer day the leaves 

 become flaccid, and the existence of a negative pressure in the 

 vessels of the stem can be ascertained. In the evening, when the 

 activity of transpiration is diminished, but active absorption of 

 water from the warm soil by the roots continues, the leaves 

 become turgid, and water gradually accumulates in the vascular 

 tissue. During the night tliis accumulation of water in the 

 vascular tissue goes on until it becomes quite full, so that there 

 comes to be not only no negative pressure, but a positive 

 pressure. This positive pressure, were there no means of re- 

 lieving it, might become injurious to the tissues; but it is re- 

 lieved by the filtering of drops out of the closed terminations of 

 the vascular bundles in the leaves, these drops making their way 

 to the surface through openings over the ends of the bundles, 

 which are either the ordinary stomata, or the specially-modified 

 water-stomata. A row of such drops on the margin of the leaves 

 may be observed in many plants in the early morning. It 

 appears, then, that during the day the loss of water by transpira- 

 tion is greater than the supply by absorption, whereas during the 

 night the contrary is the case. 



With regard to the physiological significance of transpiration, 

 it is important in that it causes a rapid current of liquid, the 

 transpiration-current, to flow through the plant from the roots to 

 the transpiring organs, more especially the leaves. This ensures 

 the distribution, not only of the absorbed water, but also of the 

 substances absorbed in solution from the soil. It will be noticed 

 that the conditions which promote transpiration, namely, light 

 and warmth, are just those which are most favourable to the per- 

 formance of their anabolic proces.ses by the organs which contain 

 chlorophyll. Thus, when the leaves are actively producing organic 

 substance, they are actively transpiring, and they are therefore 

 constantly receiving supplies of the substances absorbed from the 

 soil, substances some at least of which are essential to the 

 chemical processes in operation. Transpiration has, then, an 

 important bearing upon nutrition. There seems to be, in fact, an 



