[Chap. VIII THE TISSUE SYSTEM OF LEAVES 73 



bundles of the stem. Near the junction of the petiole and stem there is 

 a short region in which the sclerenchyma is either less or absent, and 

 in which parenchyma cells rich in cytoplasm fomi a disk-shaped layer 

 several cells thick across the petiole, except in the bundles. This is the 

 absciss layer (Fig. 35). It is formed during leaf development and may 

 be readily distinguished in longitudinal sections of the petiole from the 

 cell layers above and below it. Under various conditions — such as 



Fig. 35. Vertical sections of the bases of two petioles of coleus leaves: A, in 

 which the absciss layer is almost fully developed, and B, in which abscission is 

 nearly completed. Photomicrographs by R. M. Myers. 



drought, injury to the blade, low and high temperatures — chemical 

 changes are induced in these cells. The middle lamella and sometimes 

 other layers of the cell walls become jelly-like or are dissolved, and the 

 petiole is separated from the stem. The lea\ es of deciduous trees may be 

 supported for a time by the vascular strands, but these are ultimately 

 broken. The breakdown of the absciss layer in the petiole of some 

 herbaceous plants may take place and the leaf abscise within 48 hours. 

 Abscission is usually preceded or followed by changes in the cells at- 

 tached to the stem below the absciss layer. These changes result in 

 the closure of the vessels and the development of scar tissue evident in 

 the leaf scar. 



