472. 



THE QUARTERLY REVIEW OF BIOLOGY 



FACTORS AFFECTING CHLOROPHYLL 



Chlorophyll does not usually develop 

 in leaves grown in the dark, as illustrated 

 by the colorless sprouts of potatoes or 

 onions grown in a dark cellar, and it 

 gradually disappears from green parts 

 denied light as in the blanching of celery 

 when banked with earth. It is soon de- 

 stroyed by bright light, the plant remain- 

 ing green only because of its continual 

 renewal brought about by the agency of 

 light in the active plant. In autumn 

 when the plant no longer produces 

 chlorophyll it disappears, being bleached 

 to colorless products. 



On the other hand we find that in green 

 plants chlorophyll may occur not only 

 where light gains access to the living cells 

 but also in places where it seemingly can- 

 not penetrate, at any rate in any quantity, 

 for instance in the cortex internal to the 

 periderm, in the medullary rays and even 

 in the pith. Chlorophyll may occur in 

 the cotyledons of seeds before they are set 

 free from the ovary or from the cone; 

 species of Cucurbit a and Pinus are familiar 

 examples. In these cases the influence 

 of the light which may penetrate the cells 

 is not known. However, it is known that 

 if sugars and other foods are present in 

 sufficient amount, conifer seedlings, vari- 

 ous ferns and liverworts, and many algae 

 become green, even in total darkness, and 

 a number of angiosperm seedlings become 

 green in the dark if the fruits in which the 

 seeds develop are produced in the light. 



On the other hand algae may lose their 

 green color when grown in rich nutrient 

 media in the light. 



Because chlorophyll is present it does 

 not necessarily follow that photosynthesis 

 takes place, even though the requisite 

 conditions obtain. In the case of green 

 parasites it appears as if the chlorophyll is 

 not functional, or at any rate its photo- 



synthetic power is so small as to be masked 

 by the respiratory activity. Nothing 

 definite is known regarding the substances 

 which immediately precede chlorophyll 

 formation. 



Regarding the stability of chlorophyll 

 in chloroplasts, Iwanowski has found that 

 it is destroyed by light in solutions of 

 various concentration, and he found that 

 the more concentrated the solution the less 

 susceptible it is to destruction by light. 

 Chlorophyll in a true solution, regardless 

 of concentration, is more easily destroyed 

 than in the living leaf. The yellow pig- 

 ments he believed protected the chloro- 

 phyll from decomposition by light. 



Wurmser claimed that the resistance 

 shown by the chlorophyll in the living 

 cell to the destructive action of light is due 

 to its being protected by colloids. 



Very weak light is favorable for chloro- 

 phyll formation, while light of medium 

 intensity is most favorable. Famintsyn 

 exposed a part of an etiolated plant to 

 direct sunlight, while the intensity of the 

 light falling upon the remaining portion 

 was reduced by interposing sheets of 

 paper; greening always occurred first in the 

 reduced light . According to Wiesner this 

 phenomenon is to be explained by suppos- 

 ing that decomposition and formation of 

 chlorophyll occur simultaneously. In 

 light of low or medium intensity the 

 decomposition process is nearly absent, 

 while in strong light active formation is 

 accompanied by rapid breaking down of 

 the chlorophyll, which results in less 

 pronounced greening than occurs in diffuse 

 light. Wiesner has found that various 

 parts of the spectrum have different effects 

 upon the formation of chlorophyll. The 

 effect of light and of different portions of 

 the spectrum upon chlorophyll is a subject 

 of great importance. Studies should be 

 made of its effect upon chlorophyll in 



