Lee . — The Morphology of Leaf fall. 63 
the present example lignosuberization of the Protective-layer occurs previous 
to the amputation of the leaf. 
The external morphology of the leaf of Be tula, and its relation to the 
large axillary bud and the stem, are too well known to require description. 
Internally the arrangements are of the common type. Three vascular 
bundles leave the ring to supply each leaf, and the patch of sclerenchyma 
situated outside the phloem of each soon diminishes in amount and finally 
disappears before the leaf-base is reached. A little way up the petiole it 
again reappears in its former position, and is assisted in its function by the 
collenchymatous hypoderm of the petiole. 
Starch granules are very numerous in the cortex, but are absent from 
the mesophyll of the petiole. Com- 
pound crystals of calcium oxalate are 
abundant in both tissues. Periderm 
is present in the stem long before 
the commencement of the processes 
leading to the fall of the leaf. 
The first change which occurs 
in the base of the leaf consists in 
the increased abundance of the pro- 
toplasm of a layer of cells (10-20 cells 
in thickness) situated above the junc- 
tion of cortex and petiole. The 
layer thus affected is very thick 
(Text-Fig. 9, L.P.), and its proto- 
plasmic and starchy contents give it 
an air of activity which is almost im- 
mediately justified by the production 
of numerous cell-walls. These ap- r , ri _ 
r tudinal section of leaf- base just previous to 
pear singly in each cell, and are leaf-fail, 
orientated in all directions. Almost 
immediately the cell-walls of the Protective-layer begin to undergo change 
in chemical composition, the process of ligno-suberization commencing near 
the upper limit of the active area, and spreading downwards until the 
whole layer is completely changed (PI. IV, Fig. 5, L. PI). The whole 
process is quite similar to what has been described for the other examples ; 
the main mass of the wall becomes lignified, while a thin film of suberin 
appears on the inner face of each cell-wall, with the possible exception of 
the crystal cells included in the Protective-layer. The protoplasm gradually 
degenerates and disappears, leaving the ligno-suberized cells quite empty. 
Very early in the formation of the Protective-layer, tyloses appear in 
the vessels of the leaf-trace bundles at that level. They are usually not 
very numerous, and are chiefly confined to the secondary wood. Gummy 
