66 
Lee . — The Morphology of Leaf -fail 
a complication which in many examples tends to obscure the chief events 
that precede defoliation. Earl)/ in October there is a considerable massing 
of the protoplasm in the cells of the leaf-base at the level of the furrow 
shown externally, but, curiously enough, few or no starch granules are to be 
detected in this area. Very soon regular division walls appear in these 
active cells, producing a regular cambium which in section has a curved 
course running across the base of the petiole. By its activity the cambium 
(PL IV, Fig. 6, ca.) gives rise to < 5 -io rows of cells towards the 
petiole and 1-2 towards the cortex. The latter {phi) do not increase 
very much in size, but retain their cellulose walls and living contents, and 
are certainly homologous with the phelloderm of the stem ; the former, 
however, become highly suberized with the middle lamellae of lignin, and 
finally lose their living contents. These constitute the Protective-layer 
(L.P.). 
With the first change in the leaf-base tyloses appear in the vessels of 
the vascular tissue near the active area, and effectively close the vessels 
of the primary wood, though they are not very numerous in the secondary 
wood. Gummy lignin is also produced about the same time. 
During the formation of the Protective-layer the adjacent layer above 
has also become active, and has undergone numerous divisions by walls 
which run in all directions (PI. IV, Fig. 6, S.L.). The protoplasm first 
increases in amount, and starch granules are produced ; this is followed by 
the appearance of 3-6 irregular walls in each cell of this layer, which is 
two, sometimes three, cells in thickness. The primary walls begin to swell, 
the middle lamellae become gelatinous and finally disappear, leaving the 
leaf free to fall as soon as the rupture of the vascular tissue is accomplished. 
The separation usually commences near the dorsal surface and spreads 
rapidly across. A portion of the Separation-layer invariably remains on 
the surface of the scar ; the walls of the cells undergo ligno-suberization, 
their contents disappear, and the collapsed cells form a ragged outer 
membrane to the scar tissue. No ligno-suberization occurs in the vascular 
bundles previous to leaf-fail. 
It is singular, in view of the elaborate preparations made to bring 
about defoliation and subsequently to provide adequate protection for the 
exposed tissue, that in a remarkably small proportion of the leaves yet 
examined have these modifications been availing. A well-formed Separa- 
tion-layer is invariably present at the time of leaf-fall, yet — due possibly to 
the omission of some internal factor, or perhaps to bad climatic conditions 
which bring about leaf-fall before, so to speak, the leaf is quite ready for it — 
it is often ineffective, and separation takes place by the rupture of the thin 
lateral walls of the Protective-cambium. There is no evidence of a gradual 
elongation of the lateral walls such as Tison describes for Aristolochia 
Sipho , and it is not likely that such occurs, for when gently cutting longi- 
