330 GROWTH OF PLANTS 



of rapidly increasing importance as the temperature of the leaf rises above 

 that of the surrounding air. It seems that the dispute over the importance 

 of transpiration as a cooling factor for the green leaf will be readily dis- 

 solved by answering two questions about the physics of the green leaf 

 under illumination: How fast and to what temperature would the green 

 leaf be heated if the cooling by both transpiration and thermal emissivity 

 could be eliminated during illumination, and what part does each cooling 

 factor play under various conditions? 



Environmental Conditions and the Development of Chlorophyll 



Pigments 



A study 1^ was made of the effect of environmental conditions previously 

 considered in this chapter on the amount and ratios of the five chloroplast 

 pigments: chlorophyll a and b, carotene (c), xanthophyll (x), and a brown 

 chloroplast pigment. Improved methods were developed for separating 

 chlorophyll a and h that gave much higher yields of each; for determination 

 of carotinoids; for preparing carotene from carrots; and for determining 

 the brown pigment of the chloroplast. 



Leaf tissue can be preserved in the frozen condition without loss of chloro- 

 phyll, but the a/h ratio decreases. Increasing the CO2 concentration of the 

 air or the duration of illumination during growth decreases the chlorophyll 

 and carotinoid content of the leaf; the combination of both gives a still 

 greater decrease, all without modifying the a/h or c/x ratios. Very young 

 plants, such as seedling soybean, are a partial exception to this; in these 

 increased CO2 concentration increased the chlorophyll content. Keeping 

 plants in darkness leads to a great decrease in chlorophyll content without 

 a corresponding decrease in carotinoids, but w^th an increase in c/x ratio 

 and in the brown pigment. Continuous artificial illumination of the tomato 

 plant causes a marked reduction in chlorophyll and carotinoid in a few 

 days and a significant lowering of the a/h and c/x ratios, but a rise in the 

 brown pigment. Tomato mosaic causes a decrease in chlorophyll, which 

 may be accompanied by a decrease in carotinoids but an increase in brown 

 pigment. The a/h and c/x ratios show little change. 



Filtering the .ultraviolet rays from sunlight has no significant effect on 

 the amount of chlorophyll and carotinoids in the living leaf growing under 

 such light. Elimination of blue light gives a slight but significant decrease 

 in chlorophyll and carotinoids provided the total light intensity is kept the 

 same. Absence of the blue rays also lowers the a/h ratio. Filtering out the 

 red rays leads to an increase of both chlorophyll and carotinoids, but this is 

 partially due to reduction in total intensity. On the basis of a few experi- 

 ments, the a/b ratio seems to be higher under blue rays. Blue rays also 

 favor the development of the brown pigment. 



Reduction of the light intensity to 12 per cent of full sunlight results in 

 an increase of both chlorophyll and carotinoids without change in a/h and 

 c/x ratios, but with a reduction in the bro^vn pigment. This agrees with 



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