Seedling Structure of certain Centrospermae. 191 
half-bundles derived from c. 2 may unite with the corresponding strands 
from the other cotyledon, immediately before, or simultaneously with, 
the bifurcation of the latter (Diagram 5, which compare with Figs. 4 and 5 
of Diagram 4). Finally, the root does not always show a reduction from 
tetrarchy to diarchy. 
Abronia villosa, Wats. The vascular rearrangements resemble those 
of A. umbellata very closely ; in fact the only difference found was in one 
example of A. villosa , in which only the two innermost laterals fused on to 
the neighbouring halves of the divided central bundle of the large seed-leaf. 
The others, two on each side, united together to form two large strands, 
so that the result was exactly the same as in A. umbellata (Diagram 4, 
Fig- 3 )- 
In all the examples of these two species examined, the bundles from 
the larger cotyledon gave origin to three poles of the tetrarch root, whilst 
the bundles from the smaller seed-leaf gave rise to the smaller pole ; this 
irrespective of the age of the seedling. It may 
also be remarked that the continuity between 
the isolated protoxylem elements in the petioles 
of the seed-leaves and the corresponding poles 
of the root may be interrupted, so that they 
are almost if not quite impossible to trace. 
This is doubtless due to the growth in length 
either rupturing or pulling together the soft 
walls of the annular vessels. 
Allionia albida , Walt. The main features of the transition phenomena 
of this plant resemble those of Abronia, the chief differences which obtain 
being connected with the formation of those poles of the root situated in 
the intercotyledonary plane. The central bundle of each cotyledon gives 
rise, in the blade of the seed-leaf, to an isolated strand of protoxylem in the 
same way as in Abronia (Diagram 6 , Figs. 1-4). In the upper regions of 
the petiole of each seed-leaf several strands occur (Diagram 6, Fig. 5) : the 
two large bundles formed by the division of the main cotyledonary strand 
(b. 1 and b. 2) ; two or three laterals on the outer sides of each of these (a ) ; 
a few small bundles situated between the first two ( e ), and the group of 
protoxylem elements (c. 1). On tracing these downwards their number 
becomes less ; thus the small medianly placed bundles ( e ) unite with the 
adjacent large ones, the half-bundles of the main cotyledonary strand, and 
the lateral strands unite together until there are but one or two on either 
side. There is also in the centre a group of protoxylem-elements ( c . 1 and 
c. 2) which are never numerous. 
Either at, or immediately above, the cotyledonary node, or in the 
hypocotyl, the laterals a fuse on to the neighbouring half-bundles (Diagram 6, 
Diagram 5. Abronia umbellata. 
