68 



Science of Plant Life 



that may accelerate the falling of the leaves. Among these 



are low temperature, reduced light 

 intensity, and any disturbance of the 

 water relations of the plant which 

 results in internal drought. Disease 

 and insect injuries to the blade fre- 

 quently bring about abscission. 



Leaves contain food materials 

 when they fall. The materials used 

 in buUding the cell walls in a leaf 

 are lost to the tree when the leaf 

 falls, and the fallen leaves stiU re- 

 tain considerable amounts of starch, 

 sugar, and protein. In the autumn, 

 however, photosynthesis declines, 

 and the amoimt 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. 



Abscission in compound leaves. 

 In many compound leaves, like the 

 horse-chestnut, ash, and hickory, 

 abscission first takes place at the 

 base of each leaflet. Later the 

 peaole is cut off from the stem in 

 the same way. Consequently the 

 leaflets fall first and the petioles 

 later. In the king-nut hickory the 

 petiole remains attached to the tree 

 through the following year (Fig. 42). 



fiG. 42. oiifgbark hickory twig. 

 A IS tl: ; ^dd s 5'"s ol the terminal 

 bud ?t .b- pre/iius year, B sav- 

 eral pe^'cle remiiiunn 'itt.v-hed 

 after leaf a U and c cn^ ,- ■ ninal 

 bud that will dev lop the follow- 

 ing spring. Draw^ from a speci- 

 men collected in Uecemler. 



