INTERNAL FACTORS AND PHOTOSYNTHESIS 361 



synthesis in the leaves of the higher species of plants. In other words, it 

 appears that chlorophyll content of the leaves is seldom the limiting factor in 

 photosynthesis in such species even when all external conditions are favorable 

 for the process. 



Results somewhat at variance with the above have been obtained by Emer- 

 son (1929) and Fleischer (1935) working with the unicellular alga Chlorella. 

 The latter investigator controlled the chlorophyll content in different cultures 

 of this alga by limiting the quantity of iron, magnesium, or nitrogen present. 

 When the chlorophyll content was controlled by restricting the amount of 

 iron available, a direct proportional relation was found to exist between the 

 chlorophyll content and the rate of photosynthesis if no other factors were 

 limiting. A similar relation was found to exist when the chlorophyll content 

 was limited by regulating the amount of nitrogen available but lack of mag- 

 nesium was found to check the rate of photosynthesis before it checked the 

 rate of chlorophyll formation. This suggests that magnesium plays another 

 role in photosynthesis in addition to its influence on chlorophyll synthesis. 



2. Hydration of the Protoplasm. — That the hydration of the protoplasm 

 is an important internal factor influencing photosynthesis has already been 

 shown in the discussion of the water factor in photosynthesis. 



3. Leaf Anatomy. — The rate of photosynthesis in any leaf will be partly 

 conditioned by the anatomy of that leaf. The size and distribution of the 

 intercellular spaces, the relative proportions and distribution of palisade and 

 spong}' layers, the size, position, and structure of the stomates, the thickness 

 of the cuticular and epidermal layers, the amount and position of sclerenchyma, 

 proportion and distribution of non-green mesophyll tissues, the size, distribu- 

 tion and efficiency of the vascular system, etc. will all influence the rate of 

 photosynthesis. The effects of the structure of leaves upon the rate of photo- 

 synthesis result principally from influences upon the rate of entrance of carbon 

 dioxide, upon the intensity of light penetrating to chlorenchyma cells, upon 

 the maintenance of the turgidity of the leaf cells, and upon the rate of trans- 

 location of soluble carbohydrates out of the photosynthesizing cells. 



4. Protoplasmic Factors. — Evidence from various types of experiments, 

 some of which have been presented on the foregoing pages, indicates conclu- 

 sively that certain other conditions resident in the protoplasm of plant cells, 

 other than chlorophyll content and hydration, influence the rate of photo- 

 synthesis. It is not possible to say with certainty just how many such pro- 

 toplasmic factors there are, but indications regarding two of them are fairly 

 specific. The apparent necessity of oxygen for photosynthesis suggests very 

 strongly that the respiratory mechanism of the cells is in some way involved 

 in the process. Secondly, various lines of evidence suggest that enzymatic 



