64 PHOTOSYNTHESIS 



and the opposite effect is attained with COa-free air. Lloyd ^ states : "My 

 conclusion is, therefore, that the presence or absence of COo has no direct 

 influence on stomata, and that, physiologically, they are not at all dependent 

 upon photosynthetic processes within the guard-cells. If this is true, 

 the guard-cell is set off as distinct physiologically from the chlorenchyma- 

 cell. . . . The mere fact that stomata open in the absence of COo shows 

 conclusively that the movement is not directly connected with photosyn- 

 thetic activity, even if the process takes place normally in the stomata. 



"The reduction and increase of starch in the guard-cells, in the absence 

 of carbon dioxide, points rather clearly to the activity of an enzyme, 

 presumably a sort of diastase, as a factor in the mechanism. . . ." 



Undoubtedly the causes underlying the opening and closing of the 

 stomata are of a very intricate nature. In view of the fundamental 

 importance of stomata to the photosynthetic process the subject is in 

 great need of thorough investigation on the basis of experimental pro- 

 cedure in which the various factors are subject to careful control. In 

 this connection the findings of Molisch and others (discussed here in the 

 chapter on carbohydrate transformations) of the conversion of starch 

 into soluble sugars in wilting leaves may be of considerable significance. 



The old debate as to the path of gaseous exchange has been quite 

 definitely settled. The idea that the COo is absorbed through the cuticle 

 of the leaf, as maintained by Boussingault ^ and Barthelemy,^ has been 

 replaced by the establishment of the stomata as the main path of gaseous 

 exchange. By stopping-up the stomata, Mangin and Stahl ^° were able 

 to demonstrate the retardation of gaseous exchange. Quantitative rela- 

 tions were first established by Blackman ^^ who measured the quantity 

 of carbon dioxide passing in and out of living leaves of which the distribu- 

 tion of the stomata had been determined. The plugging of the stomata 

 with vaseline or water does not completely prevent carbon dioxide diffusion, 

 for Blackman found that more carbon dioxide passes through the surface 

 in which the stomata had been stopped-up with vaseline than through the 

 unvaselined upper surface containing no stomata. Under such artificial 

 conditions there is a slight diffusion of carbon dioxide through the cuticle, 

 but the results seem to indicate that under normal conditions the stomata 

 are the chief path of gaseous exchange in the leaf. Blackman used spe- 

 cially devised glass chambers which were sealed to both sides of the leaves 

 and air was passed through these chambers. By determining the difference 

 in the amount of carbon dioxide in the air before and after passing through 

 the chambers the quantity of carbon dioxide given off by the leaf was 



'Lloyd, I.e., 125. 



'Boussingault, Aqronomie, 4, 359 (1868). 



'Barthelemy, Ann. Set. Nat. Bot., V Ser., 9, 287 (1868) ; 19, 131 (1874) ; Comnt 

 rend.. 84, 663 (1877); 85. 1055 (1877). 



"Mangin, Compt. rend., 105, 879 (1887). Stahl. Bot. Zeitg., 52, 117 (1894) 

 Garreau, Ann. Sci. Mat. Bot., ITT Ser., 13, 321 (1850). Merget, Compt. rend 84 

 376, 957 (1877): 86. 1492 (1878). Wiesner and Molisch, Sit:;bcr. K Akad Whs 

 Wicn.. 79, I, 368 (1879). 



"Blackman, PhU. Tran.';. Roy. Soc. London, B., 186, 503-562 (1895). 



