86 THE PHYSIOLOGY OF STOMATA. 



closing in many plants may indeed take place under direct insolation, the 

 "protection afforded" is less than "is often imagined." Thus, Bergen has 

 found that the older leaves "of the Myrtus type" transpire more than the 

 younger (19046) and that, in a number of thick, xerophylous-leaved ever- 

 greens, the sun leaves transpire more than the shade leaves, and as much 

 or more than many mesophytes (19040). These rather unexpected results 

 were attributed by Bergen to differences in the physical conditions within 

 the leaf, such as the more highly developed vascular tissues and more exten- 

 sive evaporation surface of the sun leaves as compared with the shade leaves, 

 though it was believed by him that this explanation is not wholly adequate. 

 The conclusion was drawn, which accords well at all points of contact with 

 my own studies, that "xerophytic leaf structure is not always incompatible 

 with abundant transpiration, but sometimes exists only for use in emergen- 

 cies to protect the plant from injurious loss of water." Aside from the con- 

 sideration that shade leaves may cease to transpire before they show any signs 

 of wilting, in which, however, wilting may have proceeded far enough to 

 result in the closure of the stomata, if this did not intervene from other cause, 

 Bergen 's observations indicate the truth of Schimper 's above-quoted remark, 

 and that the stomata are not by any means as effective in suppressing trans- 

 piration as supposed. In a word, the protection of the plant as regards its 

 water content, by stomata, is chiefly or entirely confined to their closed 

 condition. 



Haberlandt (1904, p. 407) sees a difficulty in regarding the closure of 

 stomata during wilting as a simple result of the reduction of turgor within the 

 guard-cells, since, he observes, this concentration would, because of the re- 

 duction of the amount of water, lead to a higher concentration of sap, which 

 would in turn lead to opening. It is evident, however, that wilting robs all of 

 the cells of water, and this alone would lead to a concentration of solution 

 in all cells at the same time. The resultant lowering of tensions would, then, 

 be shared by the stomata, and there would result a closure. This closure may, 

 however, not be as rapid as the wilting itself, though it is not long delayed. 

 This delay may perhaps be referred to a slightly slower loss of water by the 

 stomata. If, then, in conclusion of this portion of the study, the behavior 

 of stomata in respect to regulation of water loss, both during the normal 

 daily march of events and during wilting, is such as described, it is very 

 natural to ask what is the function of these organs. The answers which have 

 been given at various times have already been indicated. More recently 

 C. E. Bessey (1898) has argued that the stomata have made possible the 

 exchange of gases which would otherwise have been suppressed by the means 

 adopted by the plants for conserving the water supply upon their emergence 

 from a watery environment, and that therefore we must regard them primarily 

 as organs for the interchange of oxygen and carbon dioxid. 



