78 Lee . — The Morphology of Leaf -fall. 
The Separation-layer is formed, without any division, from the cells of 
the petiole adjacent to the Protective-layer, and is distinguished in the usual 
way by its abundant protoplasmic and starchy contents. Later the walls 
swell slightly, and finally the middle lamellae of the outer cells disappear, 
and separation results. No lignification occurs in the cells above the 
Separation-layer. In the second year the cork cambium again becomes 
active, and taking a more horizontal direction traverses the vascular bundle. 
Eventually all the cells above become ligno-suberized, and abundant gummy 
lignin and numerous tyloses appear in the vessels near the leaf-base. 
In view of Tison’s very clear distinctions between the different types, 
a case which appeared in this example — instances of which are far from 
rare in this as well as other species — is especially interesting. On cutting 
sections of quite ordinary leaf-scars it was found that although there had 
been the ordinary vital separation, and although the Protective-layer was 
of the usual shape and well formed, there were no signs either of stem 
periderm or of Protective-periderm. That is to say, in this case the 
formation on which Tison bases his type was altogether absent, although 
apparently everything else was quite normal. Such an occurrence cannot 
be looked upon as exceptional, for in addition to more than one instance 
being discovered, all transitions from the normal were found in a very small 
quantity of material. 
Baccharis halimifolia, Linn. 
In the mature leaf, the small triangular petiole, which is hollowed out 
and winged to provide a receptable for the small axillary bud, possesses 
near its base a deep furrow which marks the position of the Separation- 
layer. Internally, extensive modifications occur in the three vascular bundles 
with which the leaf is supplied, for as they traverse the leaf-base, not only 
are the two patches of sclerenchyma near the xylem and phloem in each 
reduced to almost nothing, but the bundles themselves decrease considerably 
in bulk. Another curious feature is the entire absence of starch granules 
from the cortex and petiole. The junction between the latter is marked by 
an area of smaller cells in which just before leaf-fall the protoplasm becomes 
very abundant, and ligno-suberization commences. In the region of the 
furrow the cells become active, the walls swell and become mucilaginous, 
and finally separate from each other. 
Above the Separation-layer a distinct Lignified-layer is present before 
leaf-fall. In the fresh scar the exposed cells soon lose their living contents 
and become flattened out on the surface. Immediately beneath, the process 
of ligno-suberization, which commenced before defoliation, spreads rapidly 
(sometimes apparently as two very distinct processes) and extends into the 
vascular bundles. Later, a cambium arises beneath the Protective-layer 
and produces from 4 to 8 rows of cork before the end of the second year. 
