74 
Lee . — 77 ^ Morphology of Leaf -fall 
Pyrus floribunda, Nichols. 
Externally the junction of the petiole with the stem is marked by 
a slight groove, while internally little or no difference is apparent, either in 
the size of the cells or in the cell contents ; in both cortex and petiole, 
starch grains as well as compound crystals of calcium oxalate are uniformly 
distributed throughout. There is the usual reduction in the sclerenchyma 
at the leaf-base. Previous to leaf-fall there is no formation of Protec- 
tive-layer, the only modification being the formation of the Separation- 
layer in the petiole at a little distance above the junction of the latter with 
the stem. This is produced by the fairly regular division of 2-3 rows of 
cells, distinguished in the usual manner, in each of which 1-2 new walls 
appear. Little or no growth in size occurs ; the primary walls of the outer 
cells swell considerably, and finally the middle lamellae disappear, and the 
leaf separates and falls to the ground. 
Soon after defoliation, the cells of the Separation-layer which remain 
on the scar gradually lose their contents and become flattened, forming 
a well-defined layer which aids in protecting the underlying tissue. The 
latter also becomes changed and begins to undergo ligno-suberization, the 
cells finally losing their living contents. At the same time, divisions which 
take place beneath the Protective-layer give rise to a cambium, which 
produces a thick layer of cork before the end of the first year. 
In the second year the cork cambium continues its activity, and adds 
considerably to the cork layers. During the whole of the process, few or 
no tyloses are produced in the vessels of the leaf-trace at the level of the 
Protective-layer, but the large amount of gummy lignin which appears 
completely closes these organs. 
Ceanothus Gloire-de-Versailles. (Garden origin.) 
The leaf-fall phenomena in this species are very similar to what 
has been described for Pyrus floribunda , the chief difference being the 
absence of stem periderm for a long time after leaf-fall. In Ceanothus , also, 
the position of the Separation-layer is usually fairly high up the petiole, 
so that an appreciable amount of the latter is left on the stem when the 
leaf is cast off. 
Acer pseudo-platan us, Linn. 
In this species the usual characteristics are to be noted, i. e. the presence 
of starch granules in the cortex, simple and compound crystals of calcium 
oxalate in both cortex and petiole, the appearance before leaf-fall of a 
superficial periderm in the stem, and the complete reduction of lignified 
sclerenchyma at the leaf-base. 
