The Natural Abscission of Twigs. 85 
activity of the cambium extends across this zone it is 
followed only by the peculiar differentiation of parenchyma 
unless the basipetal impulse is particularly strong. 
It has been shown by Gill '(unpublished work) that 
practically no cambial activity is initiated on the branch axis 
by the opening of the catkin buds and consequently the 
terminal bud is the only vegetative bud in the reproductive 
branch system to set in motion the basipetal differentiation. 
It is not surprising that in such shoots which remain re- 
productive year after year, the length of new extension added 
each year becomes less and less, and that cambial activity, 
though extending across the basal zone, is not sufficiently 
vigorous to be followed by typical differentiation. 
The case of the oak has not been fully examined, but the 
abscissed branches may be very numerous. After a season 
in which the trees have been largely defoliated by caterpillar 
attack, the small abscissed twigs are seen in very large numbers, 
and similarly it appears that the slender twigs produced by 
secondary activity in warm damp summers are also readily 
abscissed. In the latter case the foliage is usually attacked 
by mildew and suffers as a result of the first frosts. In addition 
to these types of small twigs, larger twigs also fall and may be 
associated with reproduction, though this point needs confirma- 
tion. In the case of the small twigs, the facts would support 
the general conception that with poor vegetative activity the 
basipetal tendency to vascular differentiation is not sufficiently 
strong to bridge the bud scale region. The actual separation 
across the swollen base is very clean and would suggest that 
a definite absciss zone is differentiated in the swollen region, 
but so far no such layer has been recognised. 
In many trees small branches which die (usually branches 
low on the trunk and thus shaded from light) are apparently 
pushed off by the expansion of the axis as it increases in girth 
around the base of such a dead branch. The loss of these 
branches in this manner gives rise to the bare unbranched 
trunk of the tree. The bark is thrown into folds and consider- 
able force must be exerted on the branch (5). A force of this 
kind cannot be exerted to any appreciable extent in the case 
of the small vegetative and larger reproductive branches of 
Poplar, or of the twigs of Oak, as the abscissed branches are 
fresh and often fall with green leaves still upon them. In 
fact, the fallen branches look so fresh and living that their 
viability was tested by planting bases and tops of such fallen 
twigs as cuttings with controls from comparable twigs cut 
from the tree. Whilst a considerable number of the control 
twigs rooted, both vegetative and reproductive, none of those 
from fallen twigs made any growth at all. These results, 
coupled with an observation that the abscissed twigs contain 
1933 April 1 
