152 PHOTOSYNTHESIS 



very pale green or colorless with restricted development of the chloroplasts. 

 This condition continues even under conditions of high illumination in- 

 tensity. The condition of chlorosis can be caused by a number of circum- 

 stances, but probably the most common is the absence of iron. 



Willstatter and Stoll cultivated plants w^ith nutrient solutions con- 

 taining no iron. While other types of leaves also poor in chlorophyll such 

 as the light green or yellow^ varieties, autumnal and etiolated leaves, showed 

 high photosynthetic activity on the basis of their chlorophyll-content, the 

 chlorotic leaves had a very low rate of photosynthesis. Thus chlorotic 

 Heliantlius leaves with a chlorophyll-content for 10 grams fresh leaves of 

 1.9, 2.9, 3.8 mg. had a Pc respectively of 13, 21.7 and 19.5 compared to the 

 normal leaves with 11.6 mg. and a Pc of 11.5. From this it is apparent 

 that in chlorotic leaves the chlorophyll is only partially utilized and that 

 in such leaves not only is the chlorophyll-content low but other essential 

 parts of the photosynthetic mechanism are imperfectly developed. 

 Chlorosis, then, means besides insufficient chlorophyll, inadequacy in other 

 parts of the photosynthesis apparatus as well. Chlorophyll itself contains 

 no iron and we have no adequate explanation of the nature of the condi- 

 tion termed chlorosis nor of the role of iron in the activity of the chloro- 

 plasts. Benjamin Moore ^^^ has reported the presence of iron in the color- 

 less portion of the chloroplasts and considers iron essential for their for- 

 mation. His conclusions of the role of iron, are, however, not entirely 

 convincing and further investigation in this field seems necessary. In this 

 connection the observations of Curtel ^^° are of interest who found that 

 chlorotic plants have a lower rate of respiration and transpiration than 

 normal plants. 



Willstatter and Stoll also showed that while the rate of photosynthesis 

 in the extreme light-green and yellow varieties was low, the photosynthetic 

 number was very high. On the other hand, in albino or variegated leaves, 

 which do not contain the yellow pigments, the rate of photosynthesis is 

 low as is also the Pc- 



The anthocyanin pigments which in some leaves accompany the chloro- 

 phyll are apparently without direct influence on the rate of photosynthesis. 

 The photosynthetic activity of fruit skins is very similar to that of leaves. 



An important fact brought out by the investigations of Willstatter 

 and Stoll is that leaves of the light green or yellow variety, in their photo- 

 synthetic activity, are affected more by differences in light intensity, while 

 the leaves rich in chlorophyll are more affected by changes in temperature. 

 This is one of the facts which has led to the assumption of the existence 

 of an internal factor which Willstatter and Stoll consider to be of 

 enzymatic nature. They interpret varying effects of light and temperature 

 on leaves of different chlorophyll-content as follows. The excess or 

 larger quantity of chlorophyll in normal leaves does not result in a photo- 



'" Moore, Proc. Roy. Soc, E 87, 556 (1914). "Biochemistry," Longmans, Green 

 & Co., London, 1921, p. 53. 



'=* Curtel, Compt. rend., 130. 1074 (1900). 



