79 
THE NATURAL ABSCISSION OF TWIGS IN 
CANADIAN BLACK POPLAR. 
( Populus serotina Hartig ). 
MARY THOMAS, B.Sc. 
Introduction. — A comparison of saplings with adult trees 
of the same species shows a striking difference in the distribu- 
tion of twigs on the branches. The branch of the young tree 
usually has lateral branches down almost its* whole length, 
except for a short distance just above and below the ring of 
bud scale scars, where a few of the buds remain dormant and 
fail to produce twigs. On the other hand, the branches of 
the older tree only bear laterals at spaced intervals. The 
means by which this spacing is effected, as the tree gets older, 
is probably different in different species and the present paper 
is simply concerned with one type in which this clearing process 
is largely due to a natural process of abscission (i and 2). 
In England, the ground under trees of the Canadian Black 
Poplar (P. serotina Hartig) or the Oaks, in August or September 
is usually strewn with twigs which have fallen from the tree as 
the result of a clean separation which is quite distinct from a 
mechanical break. So constant is the appearance of the 
somewhat convex end of the twig and the concave scar left 
on the tree that it is clearly related to the structure of the 
twigs in this region, and the present investigation was under- 
taken to study the changes leading to abscission. The details 
have only been examined for P. serotina, though certain 
general observations suggest that the problem is similar in the 
Oak. 
Morphology. — The morphology of P. serotina is so closely 
bound up with the periodic growth processes of the tree, that it 
seems best to approach the consideration of the form of the 
twigs and the branch system generally, from this aspect. If 
the current year’s shoot is examined when in leaf, it is found 
that the lowest leaves are small, but leaves of increasing size 
then succeed one another until a maximum size is reached a 
little beyond half way along the shoot, beyond which the size 
falls off again towards the new terminal bud. Closely related 
to leaf size are many other features of shoot growth which, if 
graphically represented, would give identical curves. To 
draw attention to the more important of these, the following 
features may be mentioned. 
(a) Size of the buds in the axils of the leaves. 
(b) Vigour and length of the branches derived from these 
buds. 
( c ) Order of opening of the buds (with the additional 
1933 April 1 
