founded on the Structure of their Seedlings . 17 
the median protoxylem branch, if formed at all, is early 
suppressed, and the phloem group remains single. In the 
anomalous seedling of M. atlanticum a similar suppression led 
to the formation of a tetrarch root. The seedling with which 
we are dealing, however (A 3 of M. armenaicum ), forms a 
pentarch root owing to the increased activity of the lateral 
bundles. Their phloem groups remain undivided, and are 
continued directly downwards as the two lower groups of the 
root-stele (PL II, Fig. 6). Each xylem group, on the other 
hand, branches to right and left ; the two adjacent branches 
unite to form the lowest protoxylem group of the root, and 
ABC 
the upper xylem branches unite with the adjacent branches 
from the main bundles. 
Thus the lateral traces provide two phloem groups, one 
protoxylem group, and contribute elements to two other 
protoxylem groups of the pentarch root-stele. 
Finally, in seedling A 5 we find the lateral bundles con- 
tributing a full half of the tetrarch root-stele (Diagram II). 
The median protoxylem branches of both main bundles are 
suppressed. In M x this median ray is indicated during the 
earlier stages of the transition : in M 2 it never appears. Thus 
both main phloem groups are continued downwards into the 
root-stele unchanged. So are the two phloem groups of the 
lateral bundles, which are, however, smaller than the main 
groups (PI. II, Fig. 9). The xylem of main and lateral traces 
alike branches to right and left, adjacent branches fusing in 
C 
