214 
Hill and de Fraine x — On the 
Transition. 
Series A and B. Each fork of the bifurcated cotyledons contains two 
vascular strands, well separated one from the other (Diagram 9, Fig. 1) ; in 
the basal region of each cotyledon, below the fork, there are obviously four 
bundles arranged in two pairs (Diagram 9, Fig. 2). No union of bundles 
takes place in the cotyledons themselves, so that eight vascular strands 
arranged in four pairs enter the hypocotyl. The individuals of each pair 
of bundles, derived from the four prongs of two-forked cotyledons, continue 
to approach one another and ultimately fuse ; the union of the two phloem- 
groups takes place first, then the junction of the protoxylems is effected. 
The two groups of metaxylem elements remain separated by parenchyma- 
tous elements ; thus the wood has a V-like appearance (Diagram 9, Figs. 3 
and 4). The four bundles thus produced gradually rotate and become 
situated in a position roughly at right angles to the plane of insertion of the 
cotyledons. The actual transition takes place very slowly ; the two bundles 
derived from one cotyledon approach one another, and their protoxylems 
rotate towards the exarch position. Concurrently, each xylem-mass becomes 
more compact (Diagram 9, Fig. 5), and the corresponding portions ulti- 
mately fuse together ; finally, the opposing phloems effect a junction so 
that a diarch root results. 
Series C. This seedling differed from the rest of this species inasmuch 
as there were but three apparent cotyledons. The structure showed that 
really there were two seed-leaves, one undivided and the other one forked. 
The rearrangements of the vascular strands of this specimen differed 
from those of the previous Series, and are illustrated in Diagram 10. The 
first figure shows that one prong of the bifurcated cotyledon had one bundle 
a, and the other portion two, b and c. The other cotyledon had five bundles 
instead of four. Eight bundles entered the hypocotyl as in the foregoing 
Series, but their arrangement was different (cf. Diagram 9, Fig. 3, and 
Diagram 10, Fig. 3). In the upper region of the axis the traces b and c 
rapidly approached one another and fused (Diagram 10, Fig. 3), and, at 
a lower level, this compound bundle fused on to the strand a (Diagram 10, 
Fig. 4). Thus there were six bundles, five in one group derived from the 
undivided seed-leaf, and one derived from the forked cotyledon. The five 
strands fused together (Diagram 10, Fig. 5) and then separated out as two 
single structures (Diagram 10, Fig. 6), which behaved in the same way as 
in the previous Series and formed one pole of the diarch root. The single 
strand a 4 b + c underwent a lateral displacement, and its protoxylem com- 
menced its rotation towards the exarch position ; then, in a corresponding 
position on the other side of the hypocotyl, a new group of phloem elements 
appeared which gradually became more abundant, and were followed by 
the appearance of xylem (Diagram 10, Figs. 7 and 8). A new bundle was 
