LEAVES AND THEIR USES 257 



When cork layers begin to be formed across the leaf -stalk 

 in preparation for the fall of the leaf, the movement of sugar 

 out of the leaf is checked ; the leaf cells now contain more 

 sugar than usual, and the conditions are favorable for the 

 formation of red coloring matters. Bright sunlight also 

 favors the formation of these red substances. The brown 

 color of autumn leaves seems to be present, at least to a 

 large extent, in the cell walls, which turn brown as they die. 



268. Some Practical Uses of Leaves. Although we use 

 the leaves of many plants for food, their actual food value 

 is in most cases rather small. Most of the more solid and 

 really important articles of our diet, so far as they are sup- 

 plied directly by plants, come from the stems, roots, seeds, 

 and fruits in which plants store most of their reserve food. 

 The value of the leaves that we eat lies rather in their stimu- 

 lating and appetizing qualities. One may think in this 

 'connection of the leaves of onions, cabbage, lettuce, endive, 

 spinach, celery, parsley, spearmint, sage, and of others that 

 are used for salads, for flavoring, and in similar ways. Among 

 the leaves that are important for their stimulating properties 

 are those of tea and tobacco. The leaves of many plants, 

 such as arnica, boneset, spearmint, digitalis, horehound, 

 and witch hazel, contain substances that make them useful 

 in medicine. Those of not a few tropical plants, among 

 them some of the palms, supply fibers that are used in a 

 variety of ways. Many leaves, especially those of the 

 grasses, are also an important part of the food of many of 

 the lower animals which we in turn use for food, as beasts 

 of burden, and in other ways ; and in this indirect way leaves 

 are of very great value to man. 



