lOO 



General Botany 



Fjg. 52. Longitudinal sec- 

 tion of base of a petiole, 

 showing abscission layer. 

 The dropping of the leaf is 

 due to the softening of the 

 cell walls in this layer. 



near the base is still in growing condition after the other parts 

 are mature. In net-veined leaves all parts of the blade mature 

 at the same time. 



Leaf fall, or abscission. In the hfe of a 

 leaf the final stage is abscission, or the fall 

 of the leaf. On many temperate plants the 

 leaves remain only 5 to 8 months. On 

 evergreen shrubs and trees the leaves are 

 attached from 3 to 8 years. The first part 

 of the process of leaf fall is a phase of 

 growth, and we shall see that the tissue 

 which makes abscission possible is con- 

 structed long before the leaf falls (Fig. 52). 

 The causes of leaf fall. There are two 

 distinct stages in the process by which 

 plants drop their leaves : (i) the formation 

 at the base of the petiole of two or more 

 plates of thin-walled cells, 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 separation of the cells of the abscission layer, which is 

 brought about by the softening or dissolving of pectic com- 

 pounds in 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 

 corky layer is formed in some 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 that may 

 accelerate the falhng of the leaves. Among these are low tem- 

 perature, reduced light intensity, and any disturbance of the 



