Action of Light I49 



A curious effect of cold upon some plants is to change the 

 colour of the chlorophyll. Thus the leaves of Yew, Pine, 

 Juniper, Box, and other plants turn red or reddish brown under 

 the influence of cold. 



Light also plays an important part in the life and growth of 

 plants. We may divide the effects of light under two heads. 



1. Chemical Action of light. — This is seen in the formation 

 of chlorophyll and in the decomposition of the carbon dioxide of 

 the atmosphere, the assimilation of carbon, and the production 

 of starch and other organic compounds. These changes can 

 only take place in the light. A plant grown in perfect darkness 

 will, as we have seen, be etiolated, and no starch will be formed. 

 A very slight amount of light — the diffused light at the back 

 of a room — will be sufficient to produce the green colour ; but 

 for the production of starch much brighter light is necessary. 

 Metastasis, that is, the conversion of one kind of organic 

 material into another (such, for instance, as the conversion of 

 starch into sugar during the germination of many seeds), can 

 go on without the aid of light. A plant that contains a large 

 reservoir of food material will grow, nay even produce flowers 

 and fruit, in the dark ; losing, however, instead of increasing in 

 weight. The growth of tubers and bulbs in the dark and the 

 germination of seeds are cases in point. 



2. The Mechanical Action of Light. — Upon the develop- 

 ment of the internodes of growing shoots light exerts a retard- 

 ing influence. If a plant, such as a Broad Bean, is grown in 

 darkness, it is found that not only does it present an etiolated 

 appearance, but that the internodes are much longer than those 

 of a corresponding plant grown in the light. 



Again, if a plant is grown in a window, so that the sunlight 

 falls upon only one side of it, very soon the stem will be found 

 to bend over to the window. Such a bending is known as 

 heliotropism. It is produced by the retarding influence of the 

 light upon that side of the plant which is turned towards it. 

 In some few cases the bending is in exactly the opposite direc- 

 tion. When the bending is towards the light it is called posi- 

 tive, and when away from it negative heliotropism. 



The mechanical and chemical actions of light are brought 



