97 
Lee,— The Morphology of Leaf-fall 
plasmic and starchy contents of the cells of the leaf-base, followed by the 
production of numerous irregular walls. Ligno-suberization soon follows, 
and gradually a layer 6-12 cells in thickness becomes completely meta- 
morphosed, and forms a well-defined Protective-layer, from which proto- 
plasmic and other contents are entirely absent (PI. VI, Fig. 18, L.P.). 
More regular divisions which now take place on the petiolar side of 
the Protective-layer lead to the formation of a well-defined Separation- 
layer (S. L.). Numerous new walls appear in 2-4 rows of cells ; the primary 
walls become mucilaginous, the middle lamellae disappear, and the rupture 
of the vascular bundle is followed by the fall of the leaf. 
Tyloses are introduced into the vessels near the leaf-base at a very 
early stage, and later there is a copious production of gummy lignin. 
Before leaf-fall a cambium ( ca .) is formed by division of the cells beneath the 
Protective-layer, and soon becomes continuous across the vascular bundle 
with the phellogen of the stem. In the second year a new cambium arises, 
and by its activity produces a thick layer of cork. 
Phellodendron amurense, Rupr. 
The petiole on the whole is slender and cylindrical, but possesses 
a swollen base which completely encloses the axillary bud. A brown 
furrow marks the junction of the petiole with the stem. Numerous cells 
containing clusters of crystals of calcium oxalate, as well as large cavities 
containing mucilage, are present throughout the cortex and petiole, while 
starch granules are abundant only in the cortex. In the stem the pericycle 
possesses scattered patches of sclerenchyma, some of which accompany the 
three vascular bundles for a short distance before the latter enter the leaf. 
At the leaf-base, the sclerenchyma having been entirely lost, the bundles 
form a semicircle of vascular tissue which traverses the petiole and sup- 
plies the lamina. Isolated patches of sclerenchyma occur in both cortex 
and petiole, and a superficial periderm is present in the stem before leaf- 
fail. Throughout the process tyloses are few in number near the Protective- 
layer, but the vessels become filled at a later stage with quantities of 
gummy lignin. 
Ligno-suberization takes place in a layer of cells which runs across in 
a slanting direction from the upper extremity of the stem periderm to the 
junction of the petiole with the axillary bud. The upper portion of the 
Protective-layer, having already undergone division by irregular walls, first 
become ligno-suberized while the lower cells are still dividing, the new 
walls in the latter being more numerous than in the upper cells. As the 
divisions are completed, ligno-suberization sets in until finally the whole 
layer, including the living cells of the vascular bundle, is changed in character. 
So far as can be seen, the walls surrounding the mucilage cavities and those 
of the crystal cells undergo complete lignification. 
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