STOMATA DURING WILTING. 



85 



We may not deny that the closed condition of the stomata, such as may 

 occui as the result of the loss of water during wilting, is an advantage to 

 the plant, and this will be greater as the cuticular transpiration is smaller. 



During the ordinary life processes there may be variations of turgor within 

 certain limits without affecting the onward progress of these processes, and 

 inasmuch as we may not consider these turgor variations abnormal, we are 

 scarcely justified in regarding wilting, as Schwendener has (1881), as a 

 phenomenon quite apart from the conditions of normal vegetation. Cer- 

 tainly very many plants wilt regularly each day under the conditions under 

 which they grow, yet it is very possible, or rather probable, that the stomata 

 are of advantage in limiting to some degree the loss of water. We may there- 

 fore separate the wilted condition, an 

 abnormal one, from the activity of 

 stomata during wilting. This latter 

 we should not relegate to the same 

 category, at least until the closure 

 during wilting is shown clearly to be 

 abnormal. During the early stages of 

 wilting (see data above) the stomata 

 close gradually, during and some- 

 what more slowly than the wilting. 

 If the leaf is totally cut off from a 

 source of water, as in my experiment, 

 the wilting continues without further 

 opening; but in life, many times a 

 source of water may be great enough 

 to supply the need, if the stomata are 

 shut, till more is available. 



It would seem that the "prelimi- 

 nary opening" of Darwin is a disadvantage if it occurs, but why it should 

 occur is not at all clear. It so happens that, so far as my own experience 

 shows, it does not. To this extent at least, the stomata might be regarded 

 as better regulating mechanisms of water loss than other facts about them 

 would lead us to believe. 



Schimper (1903-4, p. 12) has said: 



Most xerophytes, also, have the property of closing their stomata when they begin to 

 wilt, and thus of considerably depressing their transpiration; this is, however, not quite 

 universal. Undoubtedly, under direct insolation, the protection offered by the closing of 

 stomata is much less than is often imagined. 



We might add that this property of closing during wilting of which Schimper 

 speaks is not peculiar at all to xerophytes, and further that, while normal 



I 2 3 4 5 



Fig. 31. — Diagram illustrating closure of stomata 

 in six different leaves wilted for periods vary- 

 ing from 1 to 6 minutes. Verbena ciliata. 



