Arum maculatum from the Seed. 41 1 
phloem characteristic of root-structure is attained. It is clear 
that the root will be triarch. 
It would be hopeless to try to follow the 6 rotation ’ of the 
xylem, the process by which the protoxylem becomes external, 
in the shapeless groups forming the crescent. But the xylem 
of the midrib shows it clearly enough. In the particular 
case we are considering, this group of xylem has assumed 
the form of a crescent, with the protoxylem-elements in the 
centre. Each horn is formed of two or three larger vessels 
(Fig. 15). This little crescent is at first placed symmetrically 
with reference to the larger one. Gradually, however, it 
rotates in such a way that one horn at last points outwards, 
the other directly inwards. It will be seen that in the figure 
this rotation has already begun ; the two larger elements 
which form the left-hand horn of the xylem-crescent are 
nearer the periphery of the section than those which form 
the right-hand horn. Ultimately the two left-hand vessels 
become completely external, and then die out. In this way 
the protoxylem-elements (px, Fig. 15) are left external to the 
remaining elements of the xylem. While this has been going 
on, all the bundles of the stele have drawn much closer 
together, and one or two larger vessels have been differentiated 
from the conjunctive tissue within them. In this way a xylem- 
plate is formed. 
It is clear from this description that the transition from 
stem to root structure has taken place according to the third 
type described by M. Van Tieghem 1 . The xylem-groups 
have remained in situ , ‘ rotating * through 180°, in order 
that the protoxylem may become external ; and the phloem- 
groups have branched to right and left, the adjacent branches 
of two groups uniting. The branching of the phloem takes 
place with great regularity in all the specimens we have 
examined. But it is merely formal to describe the process 
by which the protoxylem has become external as a rotation. 
Sometimes indeed, as in the example just described, a partial 
1 Van Tieghem, Traite de Botanique, 2nd ed., Vol. i, p. 783. 
