34 BOTANICAL GAZETTE [JULY 
abscission layer with the adjacent regions of the petiole, but also 
to follow the changes in the abscission layer itself from the time of 
its formation to the fall of the leaf. This latter process is most 
easily accomplished in Coleus by beginning with the terminal bud 
and taking the leaves as they appear in order down the stem, from 
the youngest to the oldest. On this basis the following description 
is applicable to Coleus plants growing in 4-inch pots under green- _ 
house conditions, with each plant bearing 8 pairs of leaves, and 
with the eighth pair in the process of abscissing. 
In the first 2 pairs of leaves below the terminal bud there is no 
evidence of an abscission layer. The cells in the region where 
the abscission layer is later to occur are in the enlargement period 
of growth. The formation of the abscission layer in Coleus is 
usually initiated in the third pair of leaves below the terminal bud. 
Cell divisions in 2-4 layers of cells across the base of the petiole 
of these leaves begin in the epidermal and cortical region and 
gradually extend inward, reaching the phloem about the time the 
leaves appear as the fifth pair in order below the terminal bud. In 
the sixth pair of leaves the formation of the abscission layer is 
practically completed and involves all the tissue of the petiole 
except the xylem tubes. Growth of the leaf in general ceases long 
before this period is reached. In the majority of cases the fourth 
pair of leaves below the terminal bud are fully expanded. The 
formation of the abscission layer in Coleus therefore begins a short 
time before the maturity of the leaf and continues for a considerable 
period afterward. As a rule the layer is 8—12 cells in thickness. 
These cells always remain smaller than the neighboring cells of the 
adjacent regions of the petiole and their walls are somewhat thinner. 
At the time of abscission alteration of the walls of the cells of the 
abscission layer is quite general, but a continuous plane of separa- 
tion is finally formed somewhat nearer the distal side of this layer. 
This extreme alteration of cell walls is localized in the abscission 
layer and is not found throughout the entire leaf, as was reported 
by WIESNER (18). 
While this description is true in general for Coleus plants bearing 
8 pairs of leaves, slight variations are not infrequent. The stages 
of development may be either retarded or accelerated. Under 
