650 



GROWTH PERIODICITY 



ce//s of fhe 

 ajbsc/ssion laijer 



coriQx 



coleus, begonia, and fuchsia. In most herbaceous species, however, the leaves 

 are retained even after they die, and only disappear by decay or by mechanical 

 disruption from the plant. In many herbaceous species most or all of the 

 leaves are retained until after the death of the entire shoot system. 



Three principal stages can be distinguished in the process of leaf abscission : 

 (i) the formation of an abscission layer, (2) the actual process of abscission 

 or separation of the base of the leaf or petiole from the stem which bears it, 



and (3) formation of a layer of 

 cells with suberized walls which 

 prevents desiccation of the tissues 

 under the leaf scar (Lee, 191 0- 



The abscission layer may be 

 differentiated as such weeks or 

 even months before leaf-fall occurs. 

 Typically it develops as a transverse 

 zone of parenchymatous cells at the 

 base of the petiole. This zone of 

 tissue is usually several cell layers 

 in thickness (Fig. 143). 



Leaf-fall or abscission proper 

 results from the dissolution of the 

 middle lamella and perhaps also the 

 cellulose wall of these parenchyma- 

 tous cells. The principal chemical 

 change occurring seems to be the 

 conversion of insoluble protopectin 

 to pectic acid and pectin. After 

 separation of the cells of the abscis- 

 sion layer the petiole remains at- 

 tached only by the vascular ele- 

 ments. These soon snap off under 

 the pull of gravity or the pressure 

 of the wind, and the leaf drops from 

 the plant. The fractured elements 

 of the vascular bundle usually become plugged with gums or tyloses. 



Water loss through the resulting leaf scar is prevented by the formation 

 of a layer of cells the inner walls of which are suberized, and the outer 

 lignified. Subsequently other layers of corky cells develop beneath this layer 

 of cells. Eventually these layers of corky cells which cover a leaf scar 

 coalesce with the corky layers {periderm) of the stem proper. 



cells of fh9 

 abscission laijor 



Fig. 143. Abscission layer at the base 

 of the petiole of a leaf of coleus {Coleus 

 hlumei) as shown in vertical section. 



