166 LIGHT, VEGETATION AND CHLOROPHYLL 



The electron microscope reveals in the grain a pile of 

 lamellae 0-01 to 0-02 />t thick; there would therefore be about 

 ten in each grain. 



Chlorophyll constitutes a relatively small part of the 

 chloroplast — only 8 per cent of its weight, in spinach for 

 example; the quantity of xanthophyll and carotene is about 

 one-fifth of this and constitutes a little less than 2 per 

 cent. 



The presence of these chloroplasts is directly connected 

 with that of anterior chloroplasts. 



In the lower plants the existing chloroplasts give birth to 

 new ones which enter into the daughter cell. 



. In the higher plants, the chloroplasts develop from mito- 

 chondria which are pre-chloroplasts. These pre-chloroplasts 

 are not made by the cell but originate from parent cells and 

 are brought by the cytoplasm of the pollen and especially of 

 the oosphere. If the flower develops on a branch without 

 chlorophyll, it will not be able to transmit chlorophyll and the 

 daughter plant will be devoid of it. The presence and quantity 

 of chlorophyll are influenced by a number of genes of which 

 not less than fifty have been counted in maize. In this species, 

 the pre-chloroplasts increase in size from less than 1 /x to 

 3 or 4 jLt before beginning to be covered with pigment. Chloro- 

 phyll does not normally appear until half the final size has 

 been attained. 



The Synthesis and Disappearance of Chlorophyll 



There is httle information on the way in which the 

 synthesis of chlorophyll is accompUshed. Genetics are intro- 

 duced in the last stages, for a mutated green alga, chlorella, 

 accumulates protoporphyrin, i.e., chlorophyll in which the 

 magnesium, the two alcohols and the closing of the tetra- 

 carbonate external cycle are lacking. One gene prevents the 

 synthesis from reaching its conclusion. Another arrests the 

 evolution a httle later, after the fixation of magnesium, when 

 the tetracarbonate cycle is still lacking. It may therefore be 

 supposed that two of the last stages of the synthesis of 



