60 THE PHYSIOLOGY OF STOMATA. 



of the stomata. Some half dozen other plants with white areas gave similar 

 facts. The closure of the stomata in non-green parts "in darkness and as the 

 leaf withers" is quite to be expected, and, as I show later, the absence of 

 chlorophyll from, or a very small amount in, the guard-cells indicates nothing 

 at all as regards their ability to function. 



In many instances, however, the stomata of non-green organs and of the 

 non-green portions of variegated leaves have been stated to be "chlorophyll- 

 free" or immobile, and, for either or both these reasons, non-functional 

 (Czeck, Kohl), or conversely, because of the relatively low rate of transpira- 

 tion of non-green parts, the non-functional condition of the stomata has 

 been believed to be demonstrated (Merget). Of interest in this connection is 

 Kohl's criticism of Wiesner's views, based upon the latter's experiments with 

 green and etiolated maize plants. The differences in the rates of transpira- 

 tion which he observed were referred to the presence or absence of chloro- 

 phyll and not to the condition of the stomata, which he found to be almost 

 closed in both green and etiolated plants. He noted also that although the 

 stomata of Harhvegia comosa are wide open in the dark, nevertheless the 

 transpiration rate is low. He concluded, therefore, "dass die Oeffnung der 

 Stomata die Verdunstung der Pflanze im I^ichte etwa zu verstarken vermag 

 soil nicht gelaugnet werden," but that his experiments showed the fluctua- 

 tions of transpiration rate noted by him "auf anderen Ursachen beruhen 

 musse." But Kohl denied the validity of Wiesner's observation, stating that 

 he himself found the stomata in green maize plants in all stages of opening. 

 He goes on to say "da nun an etiolirte Maizpflanzen wie der Augenschein 

 lehrt den Schliesszellen der Stomata das Chlorophyll fehlt, so kann das 

 Ausbleiben der starken Beschleunigung der Wasserverdunstung an solchen 

 Pflanzen ebensowohl von der Unthatigkeit der Spaltoffnungen beriihren." 

 Kohl, on the ground of direct observation, further denies Wiesner's state- 

 ment that the stomata of Hartwegia remain open in the dark. 



It is obvious that in this controversy, as in the other arguments previously 

 presented, the assumption is made, usually tacitly, that if the stomata are 

 "closed," which must be taken to mean nearly closed, a low rate of transpira- 

 tion must ensue, and if a high rate is observed, that the stomata are open. 



So far as I am aware, in those cases in which the stomata have lent them- 

 sdves to direct observation, the attention has been directed rather to the 

 responses of these organs to different stimuli than to their condition as 

 related to different rates of transpiration, and when this point has been raised 

 the same assumption has been made— that the amount of water transpired is 

 conditioned chiefly by the size of the stomatal openings. 



There is, however, evidence to indicate that Wiesner's view is the correct 

 one. Unger (1862) showed that when the transpiration of one surface of a 



