The Natural Abscission of Twigs. 
83 
examined by cutting radial longitudinal sections through the 
union and using a stain which picks up lignified tissues, e.g., 
safranin, together with a counter stain such as light green. 
Sections cut through the union of a one-year branch always 
show a difference in the vascular tissue just at the base of the 
branch. In this zone much of the lignified wood typical of 
the internodal region is replaced by parenchymatous tissue, 
which is conspicuous as a green band across the branch base. 
If two-year, three-year, etc., vegetative branches are examined 
Fig. 2. Branch unions in Populus serotina Hartig, with bark removed showing a 
smooth union in the vegetable branch (left) and a swollen in reproductive branches 
(centre and right), in which the woody tissues crack on drying. 
in a similar manner, the appearance of the basal region varies 
with the vigour of the branch. In those which are sufficiently 
vigorous, a union of lignified tissue across this zone is 
established relatively soon and such branches are likely to 
persist. In the weaker laterals the basal zone may fail to 
be bridged by lignified tissue in the second or even the third 
year, and obviously in such cases the basal parenchymatous 
zone will present a position of weakness, which cannot stand 
the strains to which it is subjected by the movements of the 
branch. 
This parenchymatous zone is still more striking in the 
case of the reproductive branches which never make connection 
by lignified tissues (except to a very slight extent by isolated 
1933 April 1 
