69 
Lee . — The Morphology of Leaf-fall. 
Class III(£). Populus balsamifera, Linn. 
It is not surprising, in view of their undoubted affinity, that the course 
of events connected with leaf-fail should be almost identical in the two 
genera, Salix and Populus. The species of the latter now to be described 
was examined because the plant happened to be conveniently situated, and 
also because the somewhat stouter petiole is more convenient to mani- 
pulate. Tison’s description of the leaf-fall in P. alba almost exactly tallies 
with what has been observed in the present species. 
As is commonly the case in this genus, the petiole of P. balsamifera is 
long and slender, and com- 
pressed laterally so that a 
transverse section appears 
roughly oval in outline. De- 
spite its fragile appearance 
the petiole is well supplied 
with sclerenchyma, especially 
in the upper part, where it is 
really very rigid. The three 
vascular bundles which leave 
the stem to supply the leaf 
take with them a quantity of 
the sclerenchymatous sheath 
which surrounds the vascular 
cylinder ; but the amount ac- 
companying each rapidly di- 
minishes as the leaf-base is 
approached until in the latter 
region often only a single 
strand remains (Text-fig. 12, 
Sell). The vascular bundles 
themselves also decrease in 
bulk as they traverse the leaf- 
base, but immediately afterwards each divides several times, and after much 
interesting orientation three groups of bundles emerge and enter the leaf- 
blade. Just above the leaf-base, the single strand of sclerenchyma gives 
place to a thick sclerenchymatous sheath, which becomes more pro- 
nounced the nearer it gets to the lamina. 
From the point of view of size, no distinction can be drawn between 
the cortical cells and those of the petiole. Abundant starch granules are 
present in the cortex and to a less extent in the petiole, while compound 
crystals of calcium oxalate are distributed indiscriminately in both. The 
well-formed hypodermal periderm which is present in the stem extends 
Text-fig. 12. Populus balsamifera. Longitudinal 
section of leaf-base at time of leaf-fall. Proj. /. = 
cells of petiole which occur between Protective- 
layer ( L.P .) and Separation-layer (Sep. /.). 
