228 THE MOVEMENTS OF WATER 



SECTION 37. The Relation between Transpiration and the 



Absorption of Water. 



The ratio between the amounts of water transpired and absorbed is not 

 constant but varies from time to time as external conditions alter. When 

 the loss by transpiration becomes relatively greater, the percentage of water 

 in the plant decreases. If the loss is so marked that the turgidity of the 

 living cells falls below a certain limit, the leaves droop and the plant 

 becomes flaccid. An increase in the amount of water absorbed will cause 

 the cells to become turgid again, provided that the loss of water has not 

 been so great as to injure them fatally. As absorption continues, the 

 amount of water present increases until a certain level is reached at which 

 the gain and loss just counterpoise one another, and the percentage of water 

 will remain constant at this level so long as the external and internal 

 conditions are unaltered. The rate of transpiration is markedly influenced 

 by external conditions, as well as by the character of the transpiring organs. 

 At the same time the plant can absorb more water from a humid than from 

 a dry soil, and hence in nature the percentage of water which a plant contains 

 must be continually undergoing slight variations, though these are generally 

 insufficient to produce any immediately perceptible external result. 



The various methods by which excessive transpiration is prevented (Sect. 

 38-40) are of the utmost biological importance when the supply of water is 

 limited. Certain plants, such as Scinperviintw, many of the Cactaceae, and 

 others also transpire so slowly that it may be months before cut branches 

 lose sufficient water to kill them. Such forms can hence exist in dry 

 climates or regions, even though they may be without any supply of water 

 for prolonged periods of time. Plants, which rapidly wither when cut, soon 

 die under such conditions unless they are able to survive desiccation, as is 

 the case in certain Mosses, Lichens, and Protophyta. The power of accumu- 

 lating a store of water and of existing upon this supply for a certain time is 

 of great biological importance in the members of the Crassulaceae and 

 various bulbous and tuberous plants, for they are thereby enabled to 

 survive periods of drought l . Indeed, in many cases special tissues for 

 water-storage may be developed, as, for example, in the leaves of Pepe- 

 romia, and these when necessary supply water to the green tissues and 

 keep them turgid 2 . The causes which regulate the movements and 

 distribution of water throughout the plant as a whole are also responsible 



1 For examples see Haberlandt, Physiol. Anat., 1896, 2. Aufl., p. 347 ; Volkens, Flora d. agypt. 

 \Yiisle, 1887, p. 52 ; Goebel, Pflanzenbiol. Schilderungen, 1889, I, p. 25. 



2 Westermaier, Jahrb. f. wiss. Bot., 1884, Bd. xiv, p. 43 ; Haberlandt, 1. c. ; Volkens, 1. c., &c. 

 Pfitzer (Jahrb. f. wiss. Bot., 1872, Bd. vm, p. 16) was the first to recognize the importance of 

 water-tissues. Every tissue can function, to a greater or less extent, in the manner indicated. On 

 collenchyma, sec C. Miiller, Bcr. d. Bot. Ges., 1890, p. 164. 



