112 



BIOLOGY AND HUMAN LIFE 



90. Light and leaves. In the absence of Hght, chlorophyl is 

 inactive and the process of starch-making is suspended. More- 

 over, if a plant is kept in dark- 

 ness 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. When we compare 

 the outer leaves of a head of 

 lettuce or cabbage with the 

 inner leaves, we see a differ- 

 ence in the amount of green 

 pigment, which illustrates the 

 same principle. 



Experiments on light in rela- 

 tion to photosynthesis show that 

 it is quite possible for plants to 

 carry on this work under artificial 

 light. By the use of strong elec- 

 tric lights it has been 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. 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 w^orking continuously, as they apparently have no need 

 for rest or sleep. More recently a crop of wheat was harvested in Minne- 

 sota, having developed "from seed to seed" in continuous artificial light. 



91. Uses of leaves. Leaves are the original sources of most 

 of our food. The leaves of many plants are of use to us directly. 

 Some are eaten, as, for example, cabbage, lettuce, spinach, 

 water cress, dandelion. The leafstalks of rhubarb and celery 

 are also used as food, although they do not contain very much 



Fig. 64. Breathing holes of plants 



j,stomates,or breathing pores, on the sur- 

 face of a leaf, inclosed by the " guard 

 cells." 2, section through a leaf, showing 

 an air space just inside the guard cells. 

 Stomates are found in the epidermis of 

 twigs as well as on leaves. As the stem 

 grows tougher the breathing holes become 

 larger and more irregular patches con- 

 necting the spaces between the cells and 

 the outside atmosphere. The roughened 

 breathing spaces on the bark ^xtlenticels. 

 3, lenticels on the bark of birch. (Mi- 

 croscopic views about X 200) 



