90 
Lee, — The Morphology of Leaf-fall . 
striking. The crystal cells do not undergo division, and only lignin can be 
detected in their walls. 
The development of tyloses is the important factor in the closure 
of the vessels at the level of the Protective-layer. At a very early stage 
they appear in the vessels of the primary, and sometimes in the secondary 
wood, and later are accompanied by a varying amount of gummy lignin. 
As soon as the Separation-layer has become well marked the adjacent 
petiolar cells, some of which have divided, become highly lignified, and 
finally lose most of their contents and exhibit great rigidity. 
Separation takes place by the disappearance of the middle lamellae of 
the outer cells of the Separation-layer. In late autumn, after the leaf has 
fallen, the cortical cells adjacent to the Protective-layer become active and 
begin to undergo regular division, the result of which, however, is not mani- 
fest until the following year, when by their activity a thick layer of cork is 
added to the Protective-layer. The phellogens of the stem and Protective- 
layer soon merge into each other, and the layers of cork subsequently 
produced are identical in form and reactions. 
For closure of laticiferous tubes see description of Morns and PI. VI, 
Fig. 17. 
Morus alba, Linn., and M. nigra, Linn. 
When mature the stout, cylindrical petiole is distinguished from the 
brown stem by its green colour. Internally, there is little difference between 
the cells of the stem and those of the petiole, and while starch is present 
only in the former, compound crystals and much laticiferous tissues are 
found throughout. None of the ‘ stone ’ cells which accompany the vascular 
tissue in the stem are found to extend into the petiole. A superficial 
periderm is present in the stem before leaf-fall. 
The massing of the protoplasm in the cells of the leaf-base is followed 
by the appearance of numerous (2-8 in each cell) new walls in the latter, 
running in a direction approximately at right angles to the long axis of the 
petiole (PI. VI, Fig. 15, L.P.). Although the mother-cells increase in 
length they still retain their distinctive outline ; and while it may be said 
that the whole of the cells of this layer usually divide almost simultaneously 
it must be noted that there is often a distinct tendency for one row to divide 
at a time, the activity then passing to the next row of cells on the petiolar 
side. The final result is that a thick layer of divided cells is produced in 
which the crystal cells and epidermal cells alone have not undergone division. 
As soon as the process of division is well advanced, ligno-suberization 
commences near the upper limit of the Protective-layer, and spreading 
rapidly in all directions may even extend to the undivided cells of the 
cortex. The whole of the Protective-layer becomes uniformly suberized, 
but the lignification is more marked in the cells near the upper limit. The 
