94 
Lee. — The Morphology of Leaf-fall. 
of cells adjacent to the upper margin of the latter (S.L.). These cells are 
at once distinguished by the presence of dense protoplasm and abundant 
starch ; the new walls, numbering 2-5 in each cell, always remain thin, and 
run in a direction roughly at right angles to the length of the petiole. 
The mother-cells begin to swell, and though retaining for a long time their 
cellulosic character, the middle lamellae of the outer cells disappear, leaving 
the cells on either side perfectly free. 
The continuity of the Separation-layer across the leaf-trace bundle is 
well shown in this example, though other changes which occur in the vascular 
elements are not quite so marked 
as usual. Tyloses are far from 
numerous in the vessels at the level 
of the Protective-layer, and gummy 
lignin, though present, is never 
abundant. Even after leaf-fall the 
vascular elements do not become 
completely closured by these means, 
but this is accomplished later by the 
great crushing which takes place 
when the cells near the surface 
begin to dry up. A slight Ligni- 
fied-layer is invariably present above 
the Separation-layer. 
It is not until after the leaf has 
fallen that there is any formation of 
periderm cork beneath the Protec- 
tive-layer, and even then it is very 
variable. During the first winter 
divisions which take place beneath 
the Protective-layer lead to the 
formation of a cambium, by the 
activity of which a varying thickness of cork is produced before the second 
season (Text-fig. 17, P.pd.). 
In the following spring a curious variation from the ordinary course is 
seen to occur. Most observers will have noticed that in Cornns sp. and 
other examples, the resumption of growth of the axillary bud is the signal 
for the casting off of the adjacent scar tissue ; and it is not very difficult to 
ascertain the mechanism by means of which this is effected. About this 
time the cells of a layer situated at some distance below the Protective- 
layer become active and give rise to a new Separation-layer in all respects 
exactly similar to the first. Its action is precisely the same, and results, 
as was remarked above, in the cutting off of the scar tissue in early spring 
(Text-fig. 17, S.L.). Above the secondary Separation-layer is a well- 
Text-fig. 17. C omits sanguinea. Longitudinal 
section of leaf-scar in second spring. 
