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The Natural Abscission of Twigs. 
(each of which contains merely a staminate catkin enclosed 
by bud scales, the catkin falling after liberation of the pollen) 
occur in the axils of all the leaves. In comparatively rare 
cases one of the buds, usually one occurring just above or 
below the zone of reproductive buds, may be vegetative and 
grow out giving rise to a lateral twig, but this is not common. 
Thus the reproductive branch system may continue year after 
year without any laterals. This does not necessarily mean 
that these unbranched shoots are very long, as the new extension 
growth, added each year, becomes less and less the longer the 
particular system has been reproductive. 
A further point of morphological distinction between the 
vegetative and reproductive branches is that the latter are 
always much swollen just above their junction with the main 
axis, whilst the vegetative twigs make a smooth union. The 
swollen base may be correlated with the region of original 
bud scale scars although these soon become obliterated by the 
stretching and cracking of the bark. 
When twigs are freshly fallen, i.e., during the period 
extending from the end of July to about mid-October, they are 
found to be still living, often with green leaves and with 
apparently healthy buds. Examined morphologically their 
characters are found to agree with those of reproductive lateral 
branches. The terminal bud is vegetative, but the majority of, 
(or all) the others are catkin -containing. In every case the 
fallen reproductive lateral is found to have made very feeble 
extension growth for a few years prior to abscission. The 
separation of the branch takes place about half-way across the 
swollen base, but close examination shows that the break is 
actually below the bud scale scars, which fall with the branch. 
It is clear from the appearance of young trees that young 
and feebly grown vegetative branches also fall, leaving a clean 
scar but these are insignificant in comparison with the fallen 
reproductive branches which may be as much as 18 years of age 
and several feet in length. It is mainly twigs of the latter 
type which have been examined in this work. 
Structure of the Abscission Zone. — If the bark (cortex 
and phloem) is removed from the region of the branch union 
exposing the wood surface in vegetative and reproductive 
branches, a difference in structure of the basal part of the 
lateral is clearly shown by their behaviour on drying. The 
vegetative branch dries showing a smooth union with the 
axis, and whilst the reproductive shows a similar appearance 
along the length of the branch, the wood in the zone of about 
0.5 cms., in width and situated about 0.5 cms. from the actual 
base of the branch, becomes much cracked and contracted 
on drying (Fig. 2). 
Comparisons of the various types of branch unions are best 
The Naturalist 
