324 



PHYSIOLOGY 



towel, another for evaporation from a sponge, etc., for the rate varies always accord 

 ing to the material with which the water is in contact. 



Adhesion. — The water which is part of a plant body adheres to the 

 particles of cell wall, cytoplasm, and its inclusions, and is held with un- 

 equal tenacity according to the amount of each substance and its rela- 

 tion to water. As a rule, the greater the proportion of water in any sub- 

 stance, the less firmly it is held. The attractions between the water 

 particles and plant substance are altered when the plant is " killed." 

 Thus, if a living and a dead leaf be placed side by side in dry air, the 

 dead leaf loses its water much more rapidly than the living one, and 

 shrivels in a few hours. Probably this is in large part due to changes 

 that the cytoplasm undergoes, which we call death; but these cannot be 

 accurately described, beyond certain gross visible changes that do not 

 help us to understand the matter. 



Cytoplasmic changes. — There are many changes that the cytoplasm 

 may undergo, which, though not visible, occur in the course of daily living. 

 The nature of these changes is not known, and the precise way in which 

 they affect water loss is not known. Some of them may be produced 

 by the very diminution of the water content itself and thus at any 

 moment may operate to alter suddenly the rate of evaporation. 



A somewhat analogous action is known in the case of a number of salts which 

 form hydrates with variable quantities of water. Thus, copper sulfate forms a 

 pentahydrate, a trihydrate, and a monohydrate. In drying at 50 the pentahydrate 

 (CUSO4, 5H2O) maintains a vapor pressure of 47 mm. (mercury) as long as any 

 pentahydrate remains; then the vapor pressure suddenly drops to 30mm., that of 

 the trihydrate (CUSO4, 3H2O). With further desiccation it again suddenly falls, 

 as soon as the trihydrate is all decomposed, to 4.5 mm., the vapor pressure of the 

 monohydrate (CuS0 4 , H 2 0), and there it remains until all the water is driven off. 

 In this case there would be at each point a sudden fall in the rate of evaporation. 

 Just such sudden alterations have been observed in transpiration. 



Regulation. — To say that the living protoplast " regulates " the loss 

 of water from a plant is only to say that as the nature of the living 

 material may change, its water relations change, and the rate of evapo- 

 ration changes in consonance. But this is not " regulation " in the sense 

 of adjusting the loss to the income, so that no harm may come to the 

 plant. It is regulation only in the sense that the crystal, when heated, 

 " regulates " the loss of its component water. In both cases evaporation 

 becomes increasingly difficult, and for the plant this may avert death 

 from excessive water loss. 



