87 
Lee . — The Morphology of Leaf-falL 
been made. It is separated from the Protective-layer by one or two rows 
of cells, and although its protoplasm is rather conspicuous, little or no starch 
is present, and except in rare cases no division walls appear. The Separa- 
tion-layer first becomes well marked near the dorsal surface ; its walls begin 
to swell, the middle lamellae become mucilaginous and finally disappear, 
and the leaf is thrown off. 
During the first winter the cells beneath the Protective-layer become 
active and give rise to a cork cambium which is continuous with that of the 
stem periderm. The layer of cork produced before the second season 
is usually not very thick, and is not continuous across the vascular bundles. 
Towards the end of the second season a new phellogen arises below the first 
one, and dipping down towards the cortex crosses the vascular bundles and 
is only interrupted by the thick patch of sclerenchyma outside each leaf- 
trace. 
Text-fig. 15. Rhus typhina. Longitudinal section of leaf-base at leaf-fall. res. = resin duct ; 
res } and res . 2 = limits of ' tyloses ’ in resin duct. 
Rhus typhina, Linn. 
This species is rather interesting in the fact that in its leaf-fall 
processes it appears to combine the characters of R. cotinis and R. coriaria 
as described by Tison. The whole plant is very hairy. The stout petiole 
has a swollen base which encloses the large axillary bud. The distribution 
of starch and calcium oxalate crystals is as usual, and periderm is usually 
present before leaf-fall. Numerous resin canals surround the vascular ring 
