. Biology and human life. Biology. 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
. Biology and human life. Biology. 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 ; 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 s
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