168 LIGHT, VEGETATION AND CHLOROPHYLL 



blue light. Red plants have more chlorophyll than blue, but 

 less carotenoids — their photosynthetic capacity for the same 

 luminous energy is greater. Moreover, the radiations which 

 promote the synthesis of chlorophyll are almost identical 

 with those which promote the growth of the whole plant; it 

 is therefore difficult to dissociate the two processes. 



Temperature also has an influence. Here chlorophyll is 

 more narrowly limited than growth and is not produced when 

 the temperature is too low or too high, although growth is not 

 suppressed. The cold causes certain leaves to develop without 

 chlorophyll and sometimes they lose permanently the capacity 

 to form it. Every species has its critical maximum and mini- 

 mum temperature and its optimum for the formation of 

 chlorophyll; the two extremes seem to be +4° and +40° C. 



A rather large number of chemical elements is necessary 

 for the formation of chlorophyll. If etiolated leaves are kept 

 for two days in distilled water in the dark, they consume all 

 their glucosides. If they are then exposed to the light, some in 

 distilled water and others in a solution of glucosides, only the 

 latter form chlorophyll. Glucosides are therefore necessary, 

 but they must not be too abundant. If they are, the plant will 

 not produce chlorophyll because the need for it does not 

 exist in the presence of an excess of precisely those products 

 which chlorophyll provides; in other words, the super- 

 abundance of glucosides mobilizes the albumins and makes 

 them unavailable for the formation of chlorophyll. 



Nitrogen is necessary, and it is well known that nitro- 

 genous fertilizers produce remarkably green leaves, while a 

 deficiency of nitrogen makes them yellow because there is not 

 sufficient chlorophyll to mask the carotenoids. Nitrogen is one 

 of the constituent elements of chlorophyll and, according to 

 Roux and Husson, it enters into the synthesis of the pyrrolic 

 nuclei in the form of glutamic acid. 



Magnesium is also a constituent element and its deficiency 

 is manifested first on the old leaves by almost the same 

 symptoms as the lack of nitrogen. 



It has been known for a long time that iron is necessary 



