CHAPTER XVII 

 OUR DEPENDENCE UPON LEAVES AND CHLOROPHYL 



105. Light and leaves. We have learned that in the 

 absence of Hght the chlorophyl is inactive and the process of 

 starch-making is suspended. Moreover, if a plant is kept in 

 darkness for a longer period, the chlorophyl begins to disap- 

 pear, and in the end the leaf will be quite white. This fact 

 is used in the blanching of celery. The earth is dug up about 

 the bases of the plants to exclude the light. When we com- 

 pare the outer leaves of a head of lettuce or cabbage with the 

 inner leaves, we see a difference as to the amount of green 

 pigment which illustrates the same principle. 



Experiments on light in relation to photosynthesis show 

 that it is quite possible for plants to carry on this work 

 under artificial light. The light that we usually have in a 

 living room in the evening is hardly strong enough to affect 

 most house plants, but by the use of strong electric lights it 

 has been found possible to hasten the growth and develop- 

 ment of lettuce so as to get it on the market at least two 

 weeks earlier than could otherwise have been done. This 

 means that the plants were given daylight while there was 

 any, and were then supplied with artificial light during the 

 night. In this way plants can be kept working continuously, 

 as they apparently have no need for rest or sleep. 



Experiments have been made to find out whether the dif- 

 ferent kinds of light that together make up white light have 

 any special relation to photosynthesis. It seems that the light 

 toward the red end of the spectrum is more effective in starch- 

 making than that toward the violet end. But species of plants 

 differ from one another in this respect. 



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