Hill and de Fraine . — On the 
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differentiated as the larger, the central strand behaves in exactly the same 
way as the corresponding bundle of c. 2 to begin with ; that is to say, it 
divides into halves and isolates some protoxylem in the blade. The lateral 
bundles are not differentiated, consisting only of desmogen. Lower down 
(Diagram 4, Fig. 3), these desmogen strands fuse on to the adjacent halves 
of the central bundle, which do not divide as in the other cotyledon. Thus 
a transverse section through the top of the hypocotyl has the appearance 
indicated in Diagram 4, Fig. 4. 
The bifurcated bundles derived from the large cotyledon undergo 
a vascular rearrangement, as a result of which each pair gives origin to 
a protoxylem pole situated in the intercotyledonary plane ; then the two 
half-bundles derived from the small cotyledon fuse on to the adjacent 
Diagram 4. Abronia umbellata. 
bundles derived from the large seed-leaf, and thus a tetrarch root-structure 
is arrived at (Diagram 4, Figs. 5-8). 
For a relatively long distance downwards the intercotyledonary xylem 
poles are much the stronger, but eventually a gradual addition of proto- 
xylem elements is made to the poles in the plane of the seed-leaves, so 
that equality is ultimately arrived at (Diagram 4, Fig. 9). Reduction, 
however, may take place ; in one example, below the level of the peg, the 
intercotyledonary protoxylem rays diminished in size and disappeared, the 
corresponding phloem strands united, and a well-marked diarch root- 
structure resulted (Diagram 4, Fig. 10), 
Minor variations in these changes may occur; thus the division of the 
two half-bundles of the large cotyledon may take place in the upper part of 
the hypocotyl, and not in the petiole of the seed-leaf, and the union of the 
