264 LECTURE XIII. 



The relation of light to the formation of chlorophyll has 

 been investigated chiefly by means of experiments with etio- 

 lated seedlings, but it must not be assumed that the process 

 takes place once and for all in the life of a plant. There 

 can be little doubt that the chlorophyll of a plant is always 

 undergoing decomposition, and it is therefore necessary that 

 the formation of chlorophyll should also be continually going 

 on. Sachs has pointed out that when green plants are kept 

 for some time in darkness the leaves gradually turn yellow. 

 The explanation of this fact is this, that, in the dark, 

 the decomposition of the chlorophyll continues, a yellow 

 colouring-matter (see infra] being the product, whereas no 

 formation of fresh chlorophyll can take place. Light is 

 necessary, therefore, not only for the first formation of 

 chlorophyll in an organ, but also for the maintenance of 

 the green colour during the whole life of the organ. 



It appears probable, as Wiesner and others suggest, that 

 chlorophyll is formed from etiolin. Wiesner, amongst other 

 experiments on the subject, observed that young Pumpkin- 

 seedlings which contain no etiolin turn green, when exposed 

 to light, much more slowly than older seedlings which con- 

 tained etiolin abundantly. 



Elfving has made the interesting observation that light 

 promotes the formation of etiolin. On exposing etiolated 

 seedlings to light for short periods at a temperature which 

 was too low to admit of the formation of chlorophyll, he 

 noticed that their leaves assumed a deeper yellow colour, and 

 he ascertained that the change of colour was due to the 

 presence of a larger quantity of etiolin. He found that the 

 rays of low'refrangibility (yellow, orange, and red) are those 

 which are especially active in the process. From this we may 

 infer that the formation of etiolin is always going on in actively 

 living chlorophyll-corpuscles; this accounts for the fact men- 

 tioned in previous lectures (pp. 154, 241) that etiolin is always 

 found to be present in alcoholic solutions of chlorophyll; and 

 it is probable that chlorophyll is formed from the etiolin when 

 the conditions are favourable. 



Although light is, as we have seen, in most cases an 



