The Growth and Fall of Leaves 



lOI 



water relations of the plant which results in internal drought. 

 Disease and insect injuries to the blade frequently bring about 

 abscission. 



Leaves contain food materials when 

 they fall. The materials used in 

 building the cell walls in a leaf are 

 lost to the tree when the leaf falls, 

 and the fallen leaves still retain con- 

 siderable amounts of starch, sugar, 

 protein, and other nutrient material 

 which leach back to the soil. In the 

 autumn, however, photosynthesis de- 

 clines, and the amount of food lost 

 by a deciduous tree through leaf fall 

 is small in comparison with the quan- 

 tity that has accumulated in other 

 parts of the plant. Even the part 

 that falls to the ground is not entirely 

 lost to the plant. It is used by other 

 plants and animals, which in turn 

 produce substances that are of great 

 importance to the original plants. 

 Forest trees are in this way benefited 

 by the leaves that fall to the ground. 

 In agriculture the leaves, stems, and 

 roots of one crop are frequently 

 plowed under to improve the soil for 

 succeeding crops. 



Abscission in compound leaves. In 

 many compound leaves, hke- the 

 horsechestnut, ash, and hickory, ab- 

 scission first takes place at the base 

 of each leaflet. Later the petiole is 

 cut off from the stem in the same 



Fig. 53. Shagbark hickory twig. 

 A is the bud scales of the terminal 

 bud of the previous year; B, 

 several petioles remaining at- 

 tached after leaf fall; and C, the 

 terminal bud that will develop 

 the following spring. Drawn 

 from a specimen collected in De- 

 cember. 



