WATER CONTENT AND TRANSPIRATION 173 



may take place, a loss over which the stoma has no control. 

 Taken in connection with the fact that at the beginning of 

 wilting the stoma actually opens wider, this indicates that 

 the stomatal mechanism is no guard against wilting. The 

 stoma may remain open while the water content of the leaf 

 is decreasing rapidly. 



§ 22. Regulation by Internal Leaf Changes 



We have seen, however, that with increasing evaporating 

 power of the air there may be a decrease in transpiration, 

 and Loftfield's statement as to the controlling effect of the 

 evaporating factors must be modified in this respect. If now 

 this decrease is not due to stomatal closure we must seek 

 for some other explanation. I^ivingston and Brown (1912) 

 consider that it is directly due to the decrease in water content 

 of the leaf. They suggest that a slight drying out of the 

 walls of the mesophyll cells of the leaf, by removing water 

 from the outer surfaces and withdrawing it within the ultra- 

 microscopic pores of the wall substance, increases the force 

 with which water is held, and so decreases the rate of tran- 

 spiration. They refer to the daily fall in water content of 

 the leaf in support of their hypothesis. Determinations on 

 Physalis show that the water balance of the leaf begins to 

 be drawn on (that is the water content falls) about 9 a.m. 

 and continues to do so till 3 p.m. At noon the relative 

 transpiration begins to fall, indicating the beginning of 

 regulation by the leaf, for evaporation increases till 3 p.m. 

 In the evening the water content of the leaf begins to rise, 

 and the balance is restored about 7 p.m. Thus, after a 

 certain depletion of the water content, regulation sets in, 

 limiting transpiration to a value less than that corresponding 

 to the evaporating power of the air. E. B. Shreve (19 14) 

 supports this view. 



An experiment of Knight's has a bearing on this theory. 

 A shoot was transpiring rather more rapidly than it absorbed 

 water, so that its water content fell slightly ; its transpiration 

 also fell till it reached a rate which just balanced absorption. 



