72 
Lee. — The Morphology of Leaf fall. 
Protective-layer, and is the only process which occurs in the crystal cells 
and mucilage cavities. 
The cambium which appears below the Protective-layer before the end 
of the first year is not very active, producing only 2-5 layers of cork cells. 
In the second year, a new cambium arises at a little distance below the 
first, and becoming continuous across the vascular bundle, produces a thick 
layer of cork which is generally curved in a curious manner. An appreciable 
quantity of phelloderm is also produced. 
Quercus palustris, Muench. 
In this species the cells of the cortex are much smaller than those of 
the petiole, and are distinguished from the latter by the presence of abun- 
dant starch granules. Crystals of calcium oxalate are present in both 
regions. Groups of peculiar stone cells are found only in the cortex and 
leaf-base, and the ordinary sclerenchyma which is present outside the 
phloem of each leaf-trace bundle undergoes great reduction at the leaf-base. 
Tyloses appear in the vessels of the leaf-base long before leaf-fall, and 
are accompanied by the production of a varying amount of gummy lignin. 
The Protective-layer becomes distinguished by its abundant cell contents, 
but no other change occurs before leaf-fail. The Separation-layer is pro- 
duced by division, by approximately parallel walls, of 2-8 rows of cells, 
situated a little distance above the junction of the petiole with the axillary 
bud ; and separation occurs in the usual way. 
Previous to leaf-fall, division of the cells immediately above the 
Separation-layer is followed by a process of lignification. After the leaf 
has fallen, ligno-suberization of the Protective-layer commences near the 
exposed surface, and spreading downwards completely metamorphoses all 
the cells at that level, including the living cells of the vascular bundle. The 
crystal cells become lignified, but the lignified elements of the vascular 
bundle as well as the patches of stone cells included in the Protective-layer 
undergo no change. 
During the first winter the cambium which arises below the Protective- 
layer produces only 4-8 layers of cork cells, and does not establish any 
connexion with the stem periderm. In the second year the cambium is 
much more active and produces a thick layer of periderm. 
Ligustrum VULGARE, Linn. 
The course of events leading to, and resulting from, the fall of the leaf 
in the Common Privet is very simple. In the cortex and petiole starch 
granules are very scanty and other cell contents far from abundant. A 
single vascular bundle supplies the leaf, and the little sclerenchyma which 
at first accompanies it entirely disappears at the leaf-base. Little or no 
