OTHER FACTORS INFLUENCING PHOTOSYNTHESIS 359 



The evidence in favor of the view that the decreased hydration of the 

 protoplasm which accompanies a reduction in cell turgidity causes a decreased 

 rate of photosynthesis comes chiefly from experiments with water plants. 

 Walter (1929), for example, has studied the effect upon their rate of photo- 

 synthesis of immersing plants of Elodea canadensis in sucrose solutions of vari- 

 ous concentrations. The greater the concentration of the sucrose solution 

 the less the turgidity of the cells, the less the hydration of protoplasm and the 

 slower the rate of photosynthesis. In one experiment the rate of photosyn- 

 thesis was appreciably retarded by immersion of the plants in an 0.3 weight 

 molar solution and almost entirely stopped in an 0.7 weight molar solution 

 of sucrose with an osmotic pressure of about 18 atmos. Plasmolysis of the 

 cells occurred at a solution concentration of between 0.3 and 0.4 weight molar. 

 The rate of respiration, on the other hand, was practically unaffected by 

 sucrose solutions at any concentration up to and including i.o weight molar. 



Since Elodea canadensis is a submerged aquatic with thin leaves which 

 bear no stomates, any reduction in the rate of photosynthesis resulting from 

 a diminution in cell turgor is most likely due to direct effects upon the hydra- 

 tion of the protoplasm of the photosynthesizing cells. It is probable, how- 

 ever, that a greater reduction in protoplasmic hydration is required in most 

 land plants than in such submerged species before the rate of photosynthesis 

 is appreciably retarded. 



Other External Factors Influencing Photosynthesis. — i. Oxygen. — 

 The results of a number of investigators suggest very strongly that oxygen 

 is necessary for the process of photosynthesis. The amount required, how- 

 ever, apparently is very small as a reduction in the partial pressure of the 

 oxygen to one-hundredth of that present in the atmosphere has been found by 

 Willstatter and StoU (191 8) to have no effect upon the rate of photosyn- 

 thesis in a number of species. The apparent necessity of oxygen for photo- 

 synthesis suggests that the respiratory mechanism of the cells is probably in 

 some way involved in the process. Oxygen is probably never a limiting factor 

 for photosynthesis in plants growing under natural conditions. 



2. Mineral Elements. — Briggs (1922) has shown that the rate of photo- 

 synthesis in bean plants deficient in potassium, magnesium, iron, or phos- 

 phorus is less than in similar plants provided with all of the necessary mineral 

 elements. The mechanisms of such effects are necessarily very complex and are 

 incompletely understood, although one way in which deficiency of mineral 

 salts may influence photosynthesis adversely is by checking development of 

 chlorophyll. 



3. Various Chemical Substances. — The effects of a number of miscel- 

 laneous compounds upon photosynthesis have been studied by various investi- 



