324 PHOTOSYNTHESIS 



sugars required for starch formation in the leaves of many species is very low, 

 and has been reported to be less than 0.5 g. per lOO g. of fresh leaves in some 

 species. In the mesophyll cells of most species, therefore, starch formation 

 in the leaves quickly follows photosynthesis, much of the sugar produced in 

 the latter process being converted into starch. Under conditions favorable 

 to photosynthesis the starch content of the leaves of most species usually in- 

 creases during much of the daylight period. During the night hours the 

 starch content of leaves usually decreases due to digestion of part or all of 

 the starch back to glucose and translocation out of the cells in the form of 

 this sugar or other soluble carbohydrates produced therefrom. 



That starch synthesis is an entirely independent process from photosyn- 

 thesis is also indicated by the fact that this process does not occur in the 

 mesophyll cells of a number of species of plants, yet photosynthesis takes place 

 in these cells in the same manner as in all other green plants. Failure of the 

 leaves to synthesize starch is a characteristic feature of the metabolism of many 

 species of the LUiaceae, Amaryllidaceae, Gentianaceae, Coinpositae, and 

 Umbelliferae. 



Similarly the non-green portions of at least some kinds of variegated 

 leaves do not normally synthesize starch. However, if the sugar concentra- 

 tion within the chlorophyll-free cells is artificially increased by floating such 

 leaves on glucose solutions, starch synthesis can be induced. For the leaves 

 of the variegated geranium a glucose solution of about 0.5 M concentration 

 has been found to be optimum for the induction of starch synthesis in the 

 non-green portions (Chapman and Camp, 1932). 



The Measurement of Photosynthesis. — Three methods are in common 

 use for measuring the rate of photosynthesis : ( i ) determination of the rate 

 of oxygen release, (2) determination of the rate of carbon dioxide consump- 

 tion, and (3) determination of the rate of increase in the dry weight of 

 photosynthetically active organs. 



A Measurements of photosynthesis are complicated by the fact that certain 

 other processes involving the same materials are proceeding in the cells at 

 the same time. The process of respiration is continually in progress in all 

 cells, resulting in an oxidation of part of the carbohydrates synthesized in 

 photosynthesis. This introduces an error which is inherent in all methods 

 of measuring photosynthesis. Determinations of the quantity of photosynthate 

 produced in a given time are always less than the true value by the amount 

 of carbohydrate which has been consumed in respiration. In many measure- 

 ments of photosynthesis the simultaneous occurrence of respiration is disre- 

 garded, and the results obtained are designated as the apparent photosynthetic 

 rate, or, in other words, as the rate of photosvnthesis minus the rate of 



