Leaf Coloration and the Fall of Leaves 67 



the cells makes these tissues appear white. The ice crystals 



of which snow is composed are transparent, but the numerous 



air spaces between the crystals reflect 



the light and cause it to appear white. 



Ice likewise becomes white when it is 



filled with minute air bubbles. The 



white colors of leaves and flowers merely 



show the natural appearance of the 



parts in the absence of chlorophyll and 



other pigments. 



The causes of leaf fall. There are 

 two distinct stages in the process by 

 which plants drop their leaves : (i) the FIG. 41. Longitudinal sec- 

 formation at the base of the petiole of *? n . of ^ ase . of , petiole ' 



r showing abscission layer at 



two or more plates of thin-walled cells, beginning of leaf fall, 

 known as the abscission layer; this takes 

 place during the development of the leaves and may require 

 weeks or months for completion ; and (2) the actual separa- 

 tion of the cells of the abscission layer, which is brought 

 about by the softening or dissolving of the middle layer of 

 the walls of the abscission cells. This stage of the process 

 may take place within a few hours, or at most within a few 

 days. 



The plant is protected from disease and water loss at the 

 scars left by the falling leaves, through the addition of woody 

 and corky materials to the cell walls beneath the abscission 

 layer. This protective layer is formed in some kinds of 

 plants before the leaf drops, in other plants after the leaf has 

 fallen. 



Conditions promoting leaf fall. After an abscission layer 

 has developed, there are many climatic and soil conditions 



