[Chap. X HEREDITARY DIFFERENCES IN LEAVES 93 



flavoring confections and in medicine. Other chemicals characteristic 

 of certain leaves are the nicotine of tobacco, cocaine of coca, theine of 

 tea, and atropine of the deadly nightshade. 



Economic uses of leaves. The use of leaves as food for grazing and 

 browsing animals is of great economic importance. In connection w^ith 

 grazing animals it should be remembered that insects probably remove 

 as much or more plant materials from a pasture than the larger animals. 



The several products made from tobacco leaves are derived from dis- 

 tinct and selected varieties of the plant. The aroma, color, size, and tex- 

 ture of these leaves may be further modified both by the conditions under 

 which the plants grow, and by the different methods of curing. Each 

 variety attains its highest quality only under certain climatic conditions 

 and on certain soils. For instance, the Mammoth variety cultivated in 

 Connecticut, Maryland, and Florida has more leaves than others. The 

 Cuban variety which is extensively used for cigar wrappers is sometimes 

 shaded with cloth to increase leaf size and decrease leaf thickness. 

 Burley, a coarse-textured variety, develops best on particular soils in 

 Kentucky; it is much used in plug and smoking tobacco. Certain varie- 

 ties cultivated on the sandy soils of Virginia and the Carolinas have been 

 preferred for cigarettes. 



Grass leaves, and fibers from the agave and from a Philippine banana, 

 are extensively used for the manufacture of twine, ropes, and rugs. 



A complete list of vegetables used as food would include a number of 

 tender succulent leaves that fonn an important item of the human dietary 

 in all parts of the world. Leafy vegetables are rich in vitamins; this is 

 possibly their greatest value, aside from their contribution to flavors 

 and odors. 



A large and valuable literature deals with the effects of environmental 

 factors on heritable qualities of economic plants. In this and the preced- 

 ing chapter only a few examples have been cited. State and federal de- 

 partments of agriculture have published many bulletins dealing with 

 varieties of crop plants; with the particular temperature, light, moisture, 

 and soil conditions most favorable for their development; and with the 

 regions of the United States where they grow most successfully. 



The references below contain additional information on hereditary 

 variations in leaves, as well as in other plant organs, as indicated by 

 differences in the substances made by plants from their foods. Further 

 discussion of some of these plant products occurs in Chapter XIX. 



