53 
Lee . — The Morphology of Leaf-fail. 
in the evolution of leaf-fall structures. This may or may not be true, but in any 
case it is fairly certain that many different lines have been separately evolved. 
There is no need to describe the appearance of the leaf of Castanea 
Suffice it to say that the petiole is slender and roughly cylindrical, and 
when mature is delimited from the stem by a light brown ring. It is 
traversed by a group of vascular bundles which in the upper portion divide 
previous to entering the leaf-blade. In the stem numerous groups of 
‘stone ’ cells surround the vascular ring, and as the bundles are given off to 
supply the leaves, the respective portions of sclerenchyma accompany 
them. As the leaf-base is approached the lignification becomes less and 
less pronounced, and the ‘ stone ’ cells finally become quite cellulosic ; the 
groups also decrease in bulk, and at the leaf-base almost disappear (Text-fig. i), 
while the vascular elements also decrease in amount at the transition region. 
In the upper portion of the petiole the vascular bundles assume their former 
size, while the strengthening tissue outside the bundles increases in amount 
and again becomes completely lignified. Thus it is seen that the weakest 
part of the mature petiole is at the very base where later separation will occur. 
By cutting longitudinal sections of the leaf-base, the whole course 
of events is soon apparent. In point of size there is no difference between 
the cells of the petiole and the cortical cells of the stem, though the junction 
is marked by an area of smaller cells. Nor is there any difference in the 
cell contents : in both cortex and petiole there is a scanty supply of starch 
granules, and numerous cells contain compound crystals of calcium oxalate. 
In the mature leaf, however, the layer of smaller cells which separates the 
cortex from the petiole is evidently very active, as indicated by the great 
increase in the protoplasmic contents as well as by the manufacture and 
retention of starch granules. The vascular elements in this region also dis- 
play the results of increased activity by the presence in the vessels of numerous 
tyloses (PI. IV, Fig. i) and a quantity of a gummy substance which, from 
its property of taking the lignin stains, has been called by Tison ‘ gummy 
lignin ’. The tyloses and gummy lignin, acting together, more or less 
completely obstruct the vessels and so stop the flow of nutrient solutions 
from the stem. 
If a leaf that is just about to fall be examined, it will be seen that the 
cells on the outer side of the active area at the leaf-base are separating from 
each other, and the reason is not at once apparent. When, however, a 
sufficient number of leaves are examined, it is found that the walls of the 
outer cells of the active area first begin to swell, then become gelatinous, 
and finally the middle lamellae of a layer of cells in that region become con- 
verted into mucilage and dissolve, and the cells gradually separate from 
each other (PI. IV, Fig. i). During this period of activity the cells of the 
Separation-layer are distinguished by their abundant protoplasmic contents 
as well as by a great increase in the number of starch granules present. No 
