256 TEXTBOOK OF BOTANY 



die each year, they have no need to go to the trouble of cut- 

 ting off their leaves in order to prevent evaporation in the 

 winter. There are plants, to be sure, even in cold regions, 

 whose leaves do not drop in the fall. Conspicuous examples 

 of this sort are the pines and other cone-bearing trees, 

 which, however, are especially well protected against the rapid' 

 loss of water by the small area of their leaves, and by thick 

 layers of cutin or wax. In warmer countries, on the other 

 hand, where the ground is never frozen and there is always a 

 supply of water for the roots, a much larger proportion of 

 plants have leaves that remain alive and green throughout 

 the year. 



267. Autumn Colors. The leaves of many common 

 plants undergo changes of color in the autumn before they 

 die. The autumn tints of leaves are chiefly yellows, reds, 

 and browns, and combinations of these colors. These dif- 

 ferent colors arise in different ways. As the activities of the 

 leaf lessen at the approach of autumn, its cells cease to make 

 chlorophyl, and the chlorophyl that is present in the chlo- 

 roplasts gradually disappears. The chloroplasts always 

 contain more or less of a yellow coloring matter in addition 

 to the chlorophyl. This yellow substance does -not disap- 

 pear as rapidly as the chlorophyl does, and so when the 

 green color of the chlorophyl fades out, the yellow color 

 remains. The red colors of autumn leaves are due to sub- 

 stances which were not present earlier, but which are formed 

 in the cell sap (not in the chloroplasts) as the chlorophyl 

 disappears. These substances are similar to those which 

 give a reddish color to the young shoots of many plants in 

 the spring ; they are also like those found in the red leaves 

 of the copper beech and other plants, in many red flowers 

 and fruits, and in the root of the beet. The formation of 

 red coloring matters is favored by cold weather and by the 

 presence of sugars in the cells ; indeed, it seems probable 

 that sugars are used in their formation. 



