Hill and de Fraine . — On the 
i 76 
signs of bifurcation ; the phloem divides into two parts which gradually 
move away from one another so that the appearance of the strand, as viewed 
in transverse section, is like the letter V, the protoxylem being situated at 
the apex (Diagram 1, Fig. 2). This apparent movement is continued, hence 
the V becomes widely opened and the protoxylem comes into a central 
position and is bounded on each side by metaxylem and phloem ; in other 
words, the bundle is bicollateral (Diagram 1, Fig. 3). Whilst these changes 
have been taking place, the lateral strands have gradually approached the 
central bundle, and finally they fuse with it. The condition now obtaining 
Diagram i. Figs. 1-7 Calandrinia , Fig. 8 Portulaca. In this and in the following diagrams 
the protoxylem is indicated by black areas, the metaxylem by round or elongated dots, and the 
phloem by shading. 
is seen in the third figure of Diagram 1, which arrangement is maintained for 
some distance downwards. The seed-leaves, which have no marked petiole, 
fuse laterally to form a short tube ; at the cotyledonary node the cotyledon- 
traces are seen to have their protoxylems situated well towards the periphery, 
full exarchy being arrived at during the somewhat rapid passage towards 
the centre of the hypocotyl. In the axis a general centripetal displacement 
occurs and, concurrently, the opposing groups of phloem and metaxylem 
move towards each other and effect a junction (Diagram 1, Figs. 5 and 6) ; 
thus a diarch root-structure is organized. With regard to the fundamental 
transition phenomena, it will be observed that one bundle enters the axis 
from each cotyledon ; each of these strands bifurcates and rotates so that 
