154 PHOTOSYNTHESIS 



ratio of polysaccharides to .soluble monosaccharides, serves to emphasize 

 the important role which water plays in photosynthesis, for it is well 

 established that the accumulation of starch has an inhibitory influence on 

 photosynthesis. 



Deherain and Maquenne ^^^ as well as lljin have shown how greatly 

 the rate of photosynthesis of leaves is affected by their water-content. In 

 the investigations of lljin the rate of photosynthesis was related to the 

 degree of oj^ening of the stomata. The latter was measured by means of a 

 Darwin porometer. The methods employed cannot be considered as yield- 

 ing strictly quantitative results, but indicate that alterations in the size of 

 the stomatal openings can have a profound influence on the rate of 

 photosynthesis. 



lljin ^^° has also investigated the eft'ect of water loss in leaves on the 

 disappearance of starch from the guard-cells of the stomata. It has been 

 found that leaves which had become flaccid or wilted through loss of water 

 and had again become turgid and of normal appearance when an adequate 

 supply of water was given, did not under the latter circumstances attain 

 their original photosynthetic activity. The cause for this lies in a disturb- 

 ance of the stomatal function. The rate of photosynthesis depends to a 

 large measure upon the rate at which carbon dioxide is supplied to the 

 chloroplasts. The carbon dioxide reaches the chloroplasts through the 

 stomatal openings. If these are not open or are partially closed, obviously 

 the ingress of carbon dioxide into the leaf will be hampered and the rate 

 of photosynthesis will be decreased. Now lljin found that leaves which 

 had recovered after severe loss of water and appeared normal, still on 

 careful examination revealed the fact that a large proportion of the 

 stomata had been killed and were closed. Also, the opening and closing 

 of the stomata under normal conditions is apparently accomplished by 

 alterations in the osmotic pressure of the cell sap of the stomatal guard- 

 cells. These changes in osmotic pressure are, in part at least, due to the 

 transformation of starch into soluble carbohydrates and vice versa. Thus, 

 in a humid atmosphere, the osmotic pressure of the guard-cells increases 

 and the stomata open ; while in a dry atmosphere the soluble carbohydrates 

 are converted into starch, which can be detected in the guard-cells, the 

 osmotic pressure decreases and the stomata close. This mechanism is 

 probably dependent upon the activity of an enzyme and lljin has been 

 able to demonstrate that ample water supply favors soluble carbohydrate 

 formation while desiccation favors the formation of starch. This is quite 

 in harmony with other observations of the effect of water content on the 

 carbohydrate ratio in plants. ^"^ However, if the desiccation is carried too 

 far the activity of the enzyme is apparently impaired, consequently the 

 regulatory action of the guard-cells is inhibited and the stomata do not 



^"^ Deherain and Maquenne, Compt. rcitd.. 103, 167 (1886). lljin, Flora 116 

 306 (1923). Kreusler, Lamhv. Jahrb., 14, 951 (1885). 

 "•lljin, Jahrb. zinss. Bot., 61. 670 (1922). 

 "~Spoehr, H. A., Pub. No. 287, Carnegie Inst, of Wash., 1919, p. 57. 



