VARIATIONS IN THE WATER CONTENT OF LEAVES 289 



designate an approximately complete loss of turgor, which will be the general 

 sense in which it will be emplojed in this discussion. The longer the condi- 

 tion of wilting persists, the more pronounced such a systemic loss of turgor 

 will be. We may speak, therefore, not only of wilted leaves, or other plant 

 parts, but also of "wilted plants." 



The behavior of the stomates during wilting is of especial significance 

 because of the influence of their diffusive capacity upon the rates of photosyn- 

 thesis and transpiration. As a general rule the stomates close during wilting, 

 although in at least some species their closure is preceded by a transient widen- 

 ing of the stomatal aperture. This passing enlargement in the size of the 

 stomatal pore may be due to a more rapid loss of turgor by the contiguous 

 epidermal cells than by the guard cells, thus permitting a slight further ex- 

 pansion of the latter. Prolonged wilting has been observed to lead to a 

 reopening of the stomates of a number of species. According to Iljin (1932) 

 while a moderate decrease in the water content of a leaf causes condensation 

 of sugar to starch in the guard cells (Chap. XIII), a more pronounced 

 water loss induces the reverse reaction. Hence the diffusion pressure deficit 

 of the guard cells on wilted leaves often attains such a value that movement 

 of water occurs into them even from adjacent cells which are in a flaccid 

 condition. The resulting increase in turgor of the guard cells causes them to 

 open. 



Because of its effects on the dynamics of the internal hydrostatic system 

 and upon the stomates, wilting initiates a train of far-reaching effects upon 

 physiological conditions and processes. Even incipient wilting, in which the 

 decrease in the turgor pressure of the cells is relatively small, induces wide- 

 spread physiological effects within the plant. Some of these have received 

 attention earlier in the discussion of factors influencing the periodicity of 

 transpiration; other such effects, particularly those upon photosynthesis, wdl 

 receive consideration in later chapters. 



Daily Variations in the Water Content of Leaves. — Although a daily 

 variation in the total volume of water present in the body of a plant is a 

 common and almost regular occurrence whenever transpiration is occurrmg 

 at appreciable rates, because of the experimental difficulties involved no studies 

 have been made of this phenomenon in terms of entire plants. A number of 

 investigations have been made, however, of the diurnal variations in the water 

 content of leaves and other plant organs. 



Stanescu (1936) has studied the daily variations in the water content of 

 the leaf blades of Boston Ivy {Ampelopsis tricuspidata). The determinations 

 were made on a clear day in early November, but there is no reason for be- 

 lieving that results would be greatly different under "standard day" condi- 



