Lee . — The Morphology of Leaf fall. 
81 
Robinia Pseudacacia, Linn. 
The leaf-fail and attendant phenomena in R. Psetidacacia are quite 
similar to what has just been described for Gleditschia triacanthos ; so that 
here only the differences will be remarked. The Separation -layer is 
situated relatively higher in the petiole and so produces a more extensive 
c lip ’, which is thus able to completely enclose all the axillary buds. In its 
formation, too, there is a difference ; it is produced by numerous transverse 
divisions in a single row of cells, though sometimes it seems probable that 
more than one row takes part. 
Clerodendron trichotomum, Thunb. 
Externally the petiole is hairy and cylindrical, and passes without any 
clear distinction into the stem. The internal arrangements are slightly 
different from what may be called the common type, for in this case only two 
bundles leave the vascular ring in the stem and pass out to supply the leaf, 
though as usual the sclerenchyma accompanying each disappears before the 
leaf-base is reached. Starch in the form of granules is present in the cortex ; 
and in the groups of stone cells which are distributed in both cortex and 
petiole are seen numerous rhomboidal crystals of calcium oxalate. 
The Separation-layer is formed by the regular division of 2-3 rows of 
cells, and takes a curved course across the leaf-base. It is early defined 
by the presence of abundant protoplasm and numerous starch granules. 
Division by 2-4 walls is followed by the thickening of the primary walls, 
the middle lamellae of which disappear and allow the leaf to separate from 
the stem. No Lignified-layer is present. Before defoliation occurs the cells 
below and adjacent to the Separation-layer, without any previous division, 
begin to undergo ligno-suberization. This process, which commences near 
the dorsal surface and spreads slowly across and downwards, never becomes 
extensive before leaf-fall, the layer of tissue altered at that time varying 
from one cell thick near the ventral surface to three cells near the dorsal 
surface.- After the leaf has been cast off ligno-suberization proceeds, until 
at length a fairly thick Protective-layer has become differentiated in this 
way. Finally the adjacent cells beneath divide up regularly, and so give 
rise to a cambium which, during the first winter, produces a considerable 
amount of cork, and which continues its activity during the second season. 
Tyloses are never very numerous at any stage of the process, but 
gummy lignin is invariably abundant in the vessels near the Protective- 
layer. In the second year the Protective-cambium becomes continuous 
across the leaf-trace, all the living elements above it undergoing complete 
ligno-suberization. The few crystal (‘stone’) cells which happen to be 
included in the Protective-layer become lignified only, and often a slight 
film of lignin is to be detected surrounding the body of the crystal. 
G 
