1016 BIOLOGICAL EFFECTS OF RADIATION 



must contain some colored substance capable of absorbing visible light 

 and that this role can be chiefly ascribed to chlorophyll. At the same 

 time, a more intimate study of the other pigments of the chloroplasts and 

 of the pigments of such plants which carry on photosynthesis with other 

 pigments and a modified mechanism, e.g., the sulfur bacteria (possibly 

 survivors of another age), may contribute greatly to an understanding 

 of principles involved. 



Light is essential for the greening of most plants, but there are a 

 number of other factors of equal importance, viz., temperature, oxygen 

 supply, available carbohydrates, mineral nutrients, and notably iron. 

 The influence of the various factors has usually been studied singly. It is 

 not improbable that they are intimately interrelated, and that in order 

 to gain a more complete conception of the mechanism of chlorophyll 

 formation, it will be necessary to consider the interrelation of the factors 

 concerned. The complexity of the subject is further emphasized by 

 results obtained from the extensive genetic studies of chlorophyll 

 abnormalities (64, 64a). For the greening of virescent types Demerec (19) 

 showed that temperature is quite as important a factor as light. The 

 subject of chlorophyll formation is, of course, only ancillary to the main 

 topic here being considered and we shall confine the discussion to a brief 

 review of the influence of light in this process. 



It has been realized for a long time (139) that all plants do not require 

 light for the formation of chlorophyll, although the amount formed in 

 tlie dark is usually very small as compared with that produced in the 

 light (139, 71). Some conifers (11), ferns (99, page 159), mosses (121), 

 and algae (3, 99, 41, page 442) have been observed to produce chloro- 

 phyll in the dark. Ordinarily exceedingly low intensities of light are 

 sufficient to produce some chlorophyll in most plants. Now, it is known 

 that in respiration, besides heat, some radiations are emitted as light. 

 Although these are usually of short wave-length, may it not be possible 

 that in those plants which form chlorophyll in the dark sufficient radiation 

 is produced in this manner that the plant can form chlorophyll from its 

 own internal bioluminescence? In fact, Kostytschew (55) quotes a 

 Russian investigation to the effect that the intensity of bacterial light is 

 sufficient to produce chlorophyll in etiolated plants. 



Apparently light is not necessary for the maintenance of chlorophyll 

 in all plants, for Dangeard (15) cultured Scenedesmus acutus on a nutrient 

 solution containing 1 per cent of glucose for eight years in the dark without 

 the loss of chlorophyll. At the end of this period the algae still possessed 

 the capacity to do photosynthetic work when placed in the light, although 

 profound morj^hological changes had occurred. It is interesting that 

 with higher concentrations of organic material in the nutrient solution 

 the chlorophyll disappears, an observation which has also been made 

 by Pringsheim (91, page 8). 



