634 Sargant.—A Neiv Type of Transition from 
in which any serious difficulty of manipulation arises. As 
a rule it is a simple matter to make out the facts in good 
material at the right stage of growth. 
In the second French edition of his ‘Traite de Botanique,’ 
M. Van Tieghem has described three types of transition 
from a stem to a root-structure in the primary axis of Angio- 
spermous seedlings (1891, p. 782). Tracing the transition 
downwards from stem to root, as I am accustomed to do, 
these types are shortly as follows :— 
1. The number of xylem and phloem-bundles in the upper 
part of the hypocotyl is the same as the number of such 
bundles in the stele of the primary root. The transition 
takes place by the branching of each internal xylem-group 
to right and left of the phloem-group external to it. The 
protoxylem of each branch turns outwards during the pro¬ 
cess. The phloem-groups remain in situ , and each is now 
divided from the next by a pair of xylem-branches with 
external protoxylem. Each pair fuses into a single group, 
and the root-stele is complete. 
2. The number of xylem and phloem-groups in the upper 
part of the hypocotyl is twice that of the corresponding 
bundles in the stele of the primary root. The transition 
takes place by the fusion of the phloem-groups in pairs. The 
xylem-groups also approach each other in pairs, the proto¬ 
xylem of each group becoming external during the process. 
Thus each pair of centrifugal xylem-groups becomes a single 
group of centripetal xylem. 
3. The number of xylem and phloem-groups in the upper 
part of the hypocotyl is the same as that characteristic of 
the primary root-stele. The transition takes place by the 
branching of each phloem-group to the right and left of 
the xylem-group within it. At the same time the xylem- 
groups rearrange their elements so that those of the proto¬ 
xylem become external. Thus each centripetal xylem-group 
is divided from the next by a pair of phloem-branches. When 
a new phloem-group has been constituted from each pair of 
branches, the root-stele is complete. 
