THE NATURE OF PHOTOSYNTHESIS 151 



yellow pigments of the leaf is still unknown.^^^* It has been given a role 

 in photosynthesis as well as in the respiratory process, though most of 

 these ideas are largely hypothetical. 



The photosynthetic activity of etiolated plants, or more accurately, of 

 plants in which the chlorophyll is just developing, demonstrates the dis- 

 proportionality between photosynthesis and chlorophyll-content. Will- 

 statter and Stoll investigated the photosynthesis of etiolated leaves under 

 conditions of maximal activity, i.e. 25°, 5 per cent CO. and high light 

 intensity, about 48,000 Lux, using cultures of Phaseolus vulgaris and 

 Zea mays. The etiolated plants, as soon as the first traces of chlorophyll 

 are formed in the light, are remarkably active. For example, Phaseolus, 

 with a chlorophyll-content of 0.7 mg. per 10 grams fresh leaves, had a Pc of 

 133, while the control plants grown in light with 18.6 mg. chlorophyll 

 on the same basis showed a Pc of 9.4. In general the Pc of etiolated leaves 

 is much higher than that of young leaves which developed in the light. 

 This high photosynthetic rate of etiolated leaves, or those which have only 

 just become green, holds not only when calculated on the basis of chloro- 

 phyll-content but as well as an absolute measure. Etiolated leaves which 

 had been in the light used in the experiment 2 to 4 days, showed a higher 

 rate of photosynthesis, on the basis of either, dry weight, fresh weight 

 or area than the control plants raised in light. This greater activity of 

 the etiolated plants also holds for increase in dry weight which was about 

 twice that of the control plants, after the former had developed about 

 75 per cent of the normal chlorophyll-content. Willstatter and Stoll in- 

 terpret these facts by assuming that, while chlorophyll formation is in- 

 hibited in the dark, the development of the protoplasmic or enzymatic 

 factor is not suppressed ; in fact the absence of light appears to favor the 

 development of this factor. As a result of the accumulation or higher 

 development of this enzymatic agent in etiolated leaves, these, after they 

 develop a small quantity of chlorophyll are able to surpass leaves grown 

 in the light. 



Miss Irving ^^* has also studied the photosynthetic activity of etiolated 

 plants. She endeavored to determine whether etiolated plants were capable 

 of utiHzing for photosynthesis the carbon dioxide produced by their 

 respiration. Her results do not show a decrease in carbon dioxide evolu- 

 tion when the plants were illuminated. This is in disagreement with the 

 results of Willstatter and Stoll. It is possible that Miss Irving's results 

 may be explained on the basis that the light intensity employed in her 

 experiments was very low (light from a north window) though no 

 intensities are given. 



Another condition of interest in relation to chlorophyll-content and 

 photosynthesis is that of chlorotic plants. When plants are grown in such 

 a manner that no iron salts become available to the leaves, they remain 



"^Palmer, L. S., "Carotinoids and Related Pigments," Chemical Catalog Co., 

 1922, p. 262. 



'"" Irving, Ann. Bot.. 24, 805 (1910). Ewart, Jour. Linnean Soc, 31, 554 (1897). 



