THE NATURE OF PHOTOSYNTHESIS 155 



react to changes in the water-content of the plant. No doubt there are 

 some features of the mechanism of the movement of the stomata still 

 to be explained, but Iljin's investigations have indicated one important 

 influence of water on the rate of photosynthesis. 



In experimental work it is therefore highly important that due con- 

 sideration be given the question of the humidity of the atmosphere to 

 which the plant is subjected. In mosses and leaves possessing no stomata 

 this factor is according to some authors ^"^ not of such importance, while 

 others have found considerable variation in photosynthetic activity with 

 changes of water-content. Lichens and some mosses can be dried to a 

 powdery consistency, in which condition the power of photosynthesis is, 

 of course, temporarily lost ; on the addition of water these plants regain 

 almost immediately their photosynthetic activity. In the leaves of higher 

 plants loss of water beyond a certain point results in death, and while 

 such dried and powdered leaves still exhibit a post mortal respiration with 

 the formation of carbon dioxide the power of photosynthesis is apparently 

 permanently lost. It should be stated, however, that this question has 

 not yet been definitely answered and that by the use of proper methods 

 it may be possible to obtain photosynthesis with leaf material which has 

 previously been dried. This has been shown to be possible by the investi- 

 gations of Molisch ^"^ which are discussed in the section under Internal 

 Factors. 



Dastur,^"^ from investigations in which the starch-iodine test was 

 employed concludes that with advancing age there occurs a decay of photo- 

 synthetic activity. This is at first noticeable in the mesophyll cells of the 

 margin and intravascular regions of the leaves, supposedly the regions in 

 the leaf farther removed from the vascular system supplying water. There 

 is some question, however, whether the iodine test is reliable for deter- 

 mining photosynthetic activity especially where small differences are 

 concerned. 



For plants growing under natural conditions the humidity of the air 

 and water supply of the soil are of great importance for the photosynthetic 

 activity of the plant. These factors to a large measure determine the open- 

 ing of the stomata and consequently the absorption of carbon dioxide by 

 the leaf. In general it is the latter factor more than any other, namely, 

 the amount of carbon dioxide which the plant takes up, that determines 

 the rate of photosynthesis and hence the vegetative development of the 

 plant. In xerophytic plants, such as the cacti, which exercise special 

 economy of their water-content, the gaseous exchange is as a consequence 

 also greatly affected. This applies naturally not only to the absorption of 

 carbon dioxide for photosynthesis but as well to the absorption of oxygen 

 for respiration. The result is that these plants exhibit certain modifica- 



'" Jumelle. Rev. (fcn. Bot., 4. 168. 318 (1892). Rastit, ibid.. 3, 521 (1891). 

 T'-sson. Compt. mid.. 119. 440 (1894). Htnrici, Verli. Nattirf. Ccs. Ba.<;el., 32, 157 

 (1921). 



"'Molisch, H.. Zc-t. f. Dot.. 17. 577 (1925); Bot. Zcitg., 62, 1 (1904). 



^'^Dastur, Ann. Bot., 38, 779 (1924). 



