Seedling Structure of Gy mno sperms. III. 441 
amount of rearrangement is necessary in order to bring the protoxylem 
into its exarch position. 
As regards minor features, an endodermis does not appear until a much 
lower level has been reached, but a well-marked exodermis, two or three 
layers of cells in thickness, is conspicuous. 
The metaxylem situated between the phloem-masses of the root soon 
disappears and, at the same time, the protoxylems become tangentially 
expanded (Diagram 2, Fig. 7). It is almost unnecessary to remark that the 
smaller details of the transition region are not precisely the same in the 
different individuals examined, but the above account is typical of the 
species as regards essential features, so far as can be judged from the material 
at our disposal. 
Attention may be drawn to another seedling of this species which 
was the only one that germinated out of a number of seeds sent to us as 
M. corallipes ; its external morphology is precisely similar to that of the other 
seedlings of M. spiralis , and so also is the structure of the cotyledons, with 
the exception that the secretory cells are less numerous and the mucilage- 
ducts fewer and not so well marked. 
With regard to the transition, the number of bundles towards the 
apex of the seed-leaves is more numerous than in the other examples, there 
being ten in the larger cotyledon and eight in the smaller. These strands, 
as they are followed downwards towards the node, fuse together to form 
four, which are arranged in two pairs, in each seed-leaf. The units of 
each pair of strands fuse together, either at the base of the cotyledon or 
in the outer regions of the uppermost part of the hypocotyl, so that four 
traces are ultimately produced. These rapidly pass inwards and fuse with 
what plumular bundles may be present and form a broad U-shaped and 
compact mass of vascular tissue. This undergoes a rearrangement result- 
ing in a triangular-shaped vascular cylinder, as viewed in transverse 
section, with the protoxylems situated at the corners and surrounded by 
phloem which speedily separates into three masses so that a triarch root- 
structure results. 
As regards the part played by the cotyledonary traces in the formation 
of the root-structure, the bundles of one seed-leaf formed one pole of the root, 
while the traces of the second cotyledon produced the remaining two 
poles. 
The chief features of difference between the transition-phenomena 
of this plant and the other examples of M. spiralis are that in the former 
the number of cotyledonary bundles are more numerous, and the resulting 
root-structure is triarch instead of diarch. 
In all cases the actual transition within the axis takes place with 
extreme rapidity, hence the hypocotyl for the major part of its length 
shows a root-like structure. 
H h 2 
