492 
Watson . — On the Structure and 
In the present case, however, the whole base of the branch is cut off 
by a thick layer of secondary tissue. This layer is nearly flat, but possesses 
a depression, forming a small conical pit on its outer surface round the stele 
of the branch. 
Text-fig. 2. A diagram of the base of a ulodendroid branch, to illustrate the abscission-layer 
theory of its origin. Wood, black ; middle cortex left blank ; primary outer cortex stippled ; 
secondary cortex ruled ; abscission layer cross-hatched. 
This secondary tissue has obviously been formed by a cambium which 
has arisen in the living tissues of the base of the branch — in the inner layers 
of the outer cortex, middle cortex, and stele, without any distinction ; it cuts 
right across everything and must have rapidly led to the death of the 
branch. 
In both cases, but particularly in the case under discussion, this layer 
of secondary tissue becomes continuous with the general periderm of the 
stem, but it joins this only where the periderm of the branch turns out from 
it, so as to leave the latter as a low rim round the branch scar. 
