TRANSPIRATION AND ABSORPTION OF WATER 229 



for the localized transference of water which occurs in such cases as 

 these. The cells in which the osmotic energy is weakest arc naturally 

 the first to lose their turgidity and collapse (Sect. 34), while in some 

 cases the water-storage cells undergo an elastic contraction and so are 

 able to give off water and yet remain filled with fluid. Similarly, it is by 

 maintaining a continual difference of osmotic potential that a cut shoot is 

 able to grow at the expense of the older parts, which shrivel up as water 

 is withdrawn from them (Sect. 34). Owing to the rigidity of their walls, 

 tracheides and tracheae do not shrivel or collapse when they lose their 

 watery contents. Water is readily withdrawn from dead cells, such as the 

 vascular elements just mentioned, but as the loss of water continues the 

 contained air becomes more and more rarified, so that a correspondingly 

 increasing resistance is offered to any further removal (Sect. 34). 



We have already discussed the means by which plants obtain water 

 from the soil through their roots, and in some cases from other sources also 

 by special sub-aerial organs (Sects. 25-27). It has already been mentioned 

 that plants can absorb the required water more easily from very moist soils 

 than from comparatively dry ones ; indeed a very dry soil may actually 

 withdraw water from the plant 3 . Hence a gradual decrease in the amount 

 of water present in the soil will ultimately cause a plant to become flaccid 

 if transpiration continues. On the other hand, plants which have become 

 flaccid during a hot day recover during the night, not because the soil 

 becomes more moist, but because the cooler and damper night air causes 

 the rate of transpiration to diminish. 



The amount of water which a plant contains naturally varies as 

 the external conditions alter, and as a general rule the highest possible 

 accumulation takes place when transpiration is prevented and when a high 

 exudation-pressure is reached. The absorption of water is dependent in 

 various ways upon the external conditions, just as transpiration is. Thus 

 the presence of saline solutions in the soil must diminish the absorption 

 of water to an extent corresponding with their ' osmotic equivalency. 

 A regulatory compensation may however be possible within certain limits 

 (Sect. 24), and it has yet to be determined whether the osmotic energy 

 of the root-cells does or does not increase in a soil which is ahvays poor 

 in water, or from which water is withdrawn only with difficulty. 



The dependence of absorption upon the temperature is shown by the 

 fact that tobacco plants in pots begin to flag as soon as the temperature 

 of the moist soil falls to from 2 C. to 4 C., and become stiff and turgid 

 again when the temperature is raised and the roots actively absorb water 2 . 



1 Cf. Sect. 34. Burgerstein (Materialien zu liner Monographic d. Transpiration, 1889, 2. Th., 

 p. 50) gives a summary of the experiments performed by Sachs, Heinrich, &c.. on the influence of 

 the percentage of water in the soil upon absorption. 



z Sachs, Bot. Zeitung, 1860, p. 124. 



