76 
Lee . — The Morphology of Leaf-fall. 
Ribes nigrum, Linn. 
This species differs from R. sanguineum in that; (i) The development 
of sclerenchyma in the petiole is not so pronounced. In both the rather 
thick walled cells containing mucilage and tannin which accompany the 
vascular bundle decrease in number as the latter passes through the 
Separation-layer, (2) The chief events have a tendency to occur later 
relatively to the leaf-fall, i. e. ligno-suberization before leaf-fall is much 
rarer in R. nigrum than in R. sanguineum. 
DlOSPYRUS VIRGIN1ANA, Linn. 
The present species, adopted as a type form by Tison, has therefore 
been fully described by that author, whose observations have been entirely 
confirmed in the present research. The petiole is short and erect, and 
supplied with a single vascular bundle. Sclerenchyma is entirely absent 
from the petiole with the exception of isolated groups of stone cells. 
Starch granules and calcium oxalate crystals occur in the cells of both 
cortex and petiole ; periderm may or may not be present in the stem 
previous to leaf-fail. 
The Separation-layer, indicated externally by a furrow, is formed very 
early by the irregular division of 2-3 rows of cells situated a little distance 
from the base of the petiole, and as no increase in size occurs this layer is 
a conspicuous feature. Later the Protective-layer is formed by the ligno- 
suberization, without previous division, of the cells below the Separation- 
layer. This process commences near the periphery and adjacent to the 
Separation-layer, and passes like a wave downwards, the protoplasm and 
starch rapidly disappearing at its approach. About the same time tyloses 
appear in the larger vessels. No other change occurs until later, when the 
elements on the periphery of the vascular bundle undergo ligno-suberization, 
and the protoplasm of the Separation-layer increases in density. The cell- 
walls of the latter increase a little in thickness, while the cells themselves 
grow slightly and separate from each other. Apparently there is no pro- 
duction of mucilage, the differential growth of the cells probably accounting 
for the separation. 
At the time of leaf-fall little modification has occurred within the 
vascular bundle, tyloses and gummy lignin being almost absent. During 
the second season it is traversed by a cork layer produced by the activity 
of a cambium ; this, however, seldom attains a greater thickness than 
3-4 cells. 
Halesia tetraptera, Linn. 
Quite similar in most respects to what has been very briefly described 
above for Diospyrus virginiana is the course of events in Halesia tetraptera. 
A single vascular bundle invades the petiole. Starch is absent from the 
