Ch. Ill, 12] ECONOMICS OF LEAVES 95 



Few kinds of plants are cultivated for their leaves alone, 

 aside from foliage plants, grown in gardens for ornament. 

 Direct utility is confined to a few which happen to store 

 food, as in Cabbage, or which contain some palatable relish, 

 as in Lettuce, Spinach and other " greens," or yield some 

 special product, like Tobacco, or serve as fodder for cattle, 

 as in Grasses. Such uses, however, are insignificant in com- 

 parison with the indirect importance of leaves as the source 

 for the food and other useful substances which are formed or 

 stored elsewhere in the plant. For this reason leaves, even 

 though temporary organs of little direct economic value, 

 must all be kept in health and good photosynthetic oper- 

 ation; and thereto is much of our gardening and farming 

 practice devoted. 



For best health, leaves need ample but not too much sun- 

 light, all the carbon dioxide they can get, plenty of water, 

 some mineral salts, and air. 



In winter, greenhouse plants receive little more than a 

 fourth of the sunlight of summer, and not enough for their 

 needs. Hence house plants must be given the very best light 

 available; and good modern greenhouses are studies in 

 light-efficiency, embodying the best experience and inves- 

 tigation in direction of exposure (preferably south or south- 

 east), pitch of roof, transparency of glass, and slenderness of 

 frame. On the other hand, the full summer sun contains 

 not only more energy than plants can make use of, but often 

 much more than is good for them, particularly if in green- 

 houses, where they lack the free circulation prevailing out- 

 doors. On this account it is needful, even in spring, to shade 

 such houses by curtains, slats, matting, or paint on the 

 glass. Under light thus tempered greenhouse plants grow 

 quite as well as in full sunlight, while keeping in better 

 general health. Similarly, it has been found that some kinds 

 of crops actually thrive better under some shade, though 

 this is not wholly a matter of light, but also in part of 

 protection from hail and strong winds. Thus it is found 



