SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 657 
supply, clearly lateral. In a single remarkable example the lateral branch appeared 
to be endogenous. In addition to lateral branching, indications of dichotomy, both 
• of large and small shoots, have been found. 
The large stem in fig. 96 appears from the disposition of the tissues in the stele, 
where two distinct stellate xylems are enclosed in a common mass of phloem, to be 
preparing for dichotomous branching. It is the only specimen of the kind so 
far met with. There are other indications of the occurrence of dichotomy, however. 
Thus in the case of a lateral branch, to be described below, the series showed the 
continuation of the branch into two equal branches, although the actual division 
of the stele was not followed. The division had taken place at a level where the 
stele of the primary branch had not acquired the characteristically stellate xylem. 
It is significant that in some other examples, in which two equal steles have been 
found within the same cortex, the outlines of their xylems are suggestive of an 
early stage in the progression to the stellate type. There is, however, no direct 
evidence in these cases, one of which is represented in fig. 89, that we are dealing 
with an early dichotomy of a lateral branch. 
Another region in which the branching may have been dichotomous was in the 
fine ultimate branches. These, however, are so ill preserved, that little can be 
said beyond drawing attention to the fact that their association is suggestive of such 
a mode of subdivision. The group of small twigs in fig. 48 may be compared with 
the smaller but equally decayed twigs on the right side of fig. 47. 
The relations of lateral exogenous branches to the main stem have been more 
clearly followed in a number of examples. The lateral nature of the branch is 
shown not merely by its small size relatively to the main axis (figs. 86, 90, and 95), 
but by its vascular supply being derived from a cylindrical branch-trace separated 
from one arm of the stellate xylem. This trace is considerably larger than a leaf- trace, 
but, like it, takes a sheath of phloem as it separates from the stele. It passes more 
or less obliquely through the cortex of the main stem and enters the branch. In 
this it gradually assumes the characters of a stem-stele, the xylem becoming 
angled and stellate, and giving off leaf-traces to the leaves of the branch. 
Examples of different stages in the process thus summarised may now be 
described and illustrated. In fig. 83 the separation of a branch-trace from one arm 
of the stelar xylem is seen in a moderately small stem. In fig. 84 a branch-trace 
is present well out in the cortex. Figs. 85, 86, and 87 form a series following a 
branch- trace from the outer cortex of a large stem (fig. 85) into a small branch, the 
stele of which rapidly assumes the characters of that of a leafy shoot, the xylem 
becoming stellate (figs. 86 and 87). The base of a small lateral branch still 
connected with the parent stem is shown in fig. 95, both axes being cut transversely. 
In figs. 90 and 91 the branch is standing at right angles to the parent stem, so that 
while the latter is cut transversely the branch is followed in longitudinal section. 
The further portion of the branch in fig. 91 is shown more highly magnified in 
