1 86 



THE STOMATAL MECHANISM 



in an increase in their diffusion pressure deficit relative to that of the con- 

 tiguous cells. Water therefore moves into the guard cells, increasing their 

 turgor, which in turn leads to a widening of the stomatal aperture. Similarly 

 a decrease in the osmotic pressure of the guard cells results in a diminution 

 of their turgor and a narrowing of the stomatal aperture. 



However, certain facts suggest that the effects of light upon stomates 

 cannot be interpreted entirely in terms of the osmotic mechanism just de- 

 scribed. One of these is the rapidity with which stomatal opening occurs. 



12 

 MIDNIGHT 



12 

 NOON 



e 12 



MIDNIGHT 



Fig. 47. Daily variations in the osmotic pressure of the guard cells and epidermal cells 

 of English ivy {Hedcra helix). Data of Beck (1931). 



In some species at least the stomates open within less than a minute after 

 exposure to light. It is difficult to visualize such a rapid action in terms of 

 an enzymatic reaction, since usually such reactions occur at a relatively slow 

 rate. Some investigators consider therefore that light exerts a direct effect 

 upon the guard cells in addition to its indirect influence upon the magnitude 

 of their osmotic pressure. Little if anything is "known, however, concerning 

 the mechanism of any such reaction. 



2. Influence of the Water Factor in Sto?natal Opening and Closing. — 

 As subsequent discussion will show (Chap. XVIII) development of an internal 

 water deficit in plants is of frequent occurrence, especially on warm summer 

 days. A shrinkage in the total volume of water in a plant results in general 

 in a diminution in the volume of water in each individual cell, although all 

 cells will not necessarily be affected equally. Such a decrease in the water 

 content of the leaf cells, not sufficient to induce visible wilting, is often called 

 incipient wilting. Under such conditions the guard cells usually decrease in 

 turgor as a result of osmotic movement of water into contiguous cells. Re- 

 duction in the turgor pressure of the guard cells as a result of the diminution 

 of the volume of water in them will bring about a partial to complete closure 

 of the stomates. 



There is also some evidence that diminution in the water content of 



