CHLOROPHYLL AND ASSIMILATION 133 



reached, intense insolation may weaken the green colour ; 

 in the natural course of events the leaf yellows in the autumn 

 before its fall. The chlorophyll proper is broken up, and 

 it is likely that the important amounts of nitrogen and 

 magnesium it contains pass back into the storage organs of 

 perennial plants. The yellow tints of the autumn leaf are 

 due to the carotinoid pigments remaining, perhaps not in 

 their normal condition ; the reds are due to anthocyanin, 

 the function of which is even more obscure here than when 

 it is produced by actively functioning organs, as in the 

 copper beech or in many young shoots. Purple and 

 mauve and orange tints arise from differences in the 

 acidity of the cell sap and from combinations of anthocyans 

 and carotins. The increase in chlorophyll in the developing 

 leaf is familiar in the change from the foliage of spring with 

 its briUiant yellow-green tints to the full green colour of 

 summer. It is more strikingly seen when etiolated plants 

 are exposed to light. Seedlings grown in the dark are 

 drawn, and white or pale yellow ; illuminated, the formation 

 of chlorophyll begins in a few minutes, and progresses 

 rapidly till the maximum is reached, under favourable 

 conditions, in a few days. 



Apart from this change in chlorophyll content in the 

 history of a single leaf, great differences exist between the 

 content in normal mature leaves of different species. This 

 may be due to differences in the concentration of the pig- 

 ments in the plastids, or to different numbers or sizes of 

 plastids in equal amounts of tissue. A comparison of 

 sections through leaves, for example, of a house-leek and 

 of a cherry laurel shows a very much greater number of 

 chloroplasts in the latter. 



A good many investigations on the relation between the 

 amount of chlorophyll and the assimilating capacity of 

 different leaves have been carried out. Haberlandt attempts 

 a correlation between assimilation activity and chlorophyll 

 content. Table XXVI gives his results. 



