Seedling Anatomy of Certam Sympetalae . //. 31 1 
in a median position. In this condition, the midrib strand traverses the 
greater portion of the hypocotyl. The lateral strands pass inwards, and 
each pair, after fusing completely, separate again, and, moving laterally, 
join on to the strands in the median plane (Diagram 5, II, ill, and IV). 
There are thus formed two broad ribbons, each consisting of almost con- 
tinuous masses of xylem and phloem. Near the base of the hypocotyl the 
median protoxylem strands become really exarch, and about the same time 
the original lateral strands separate once more, and, fusing in pairs with the 
protoxylem exarch, form the intercotyledonary strand of the tetrarch root 
(Diagram 5, v). 
At this stage the tetrarch arrangement is radially symmetrical, but 
changes now occur which result in a reduction of the symmetry. The inter- 
cotyledonary xylem strands become gradually smaller and the four phloem 
masses fuse in pairs, forming two large phloem groups situate in the 
cotyledonary plane. The stele is now bisymmetrical, a symmetry which 
it retains to the root-tip. Gradually the two intercotyledonary xylem masses 
disappear, or, as in one case, they pass inwards and form the core of the 
diarch root (Diagram 5, VI). 
Seedling B. The above is the usual course of events in this species, 
but in one case the tetrarch stage, so clearly shown in all the other examples, 
is entirely omitted. From the ‘ ribbon’ stage, in which the vascular tissue 
is enormously elongated laterally, one passes straight to the diarch root by 
a simple closing in of the vascular tissue followed by fusion in pairs of the 
phloem groups. It is noteworthy that in this case no two seedlings were 
exactly the same in all details. 
Zinnia pauciflora , Linn. Seedlings 4-6 cm. long, each with an 
elongated hypocotyl and a definite cotyledonary tube. 
The transition, though not very clear, is, so far as can be determined, 
almost exactly the same as in Bidens pilosa } At the base of each seed-leaf, 
however, there are five vascular bundles, of which the very small ones near 
the margin fuse with their fellows from the other cotyledon as soon as the 
cotyledonary tube is reached. At a lower level, the fused bundles unite 
with the other laterals on the same side, so that four strands enter the 
hypocotyl. Thenceforward the transition is like that in Bidejis pilosa . 
Rearrangements in the midrib are always initiated in the lamina, and the 
region of transition is very elongated. 
Rndbeckia ample xic aulis , Vahl. Seedlings about 3 cm. long, and 
possessing petiolate seed-leaves which fuse to form a cotyledonary tube. 
The transition, which takes place at a high level, is very similar to that 
in Charieis heterophylla , the chief difference being that here the fused lateral 
strands generally disappear before they reach the centre of the hypocotyl. 
Leptosyne Douglasii i D. C. Seedlings 2-4 cm. long, very slender, and 
1 See p. 313. 
Y % 
