798 
Compton. — An Anatomical Study of 
vascular supply of the lamina (Figs, io and n). It is the same kind of 
accessory vascular strand which is of such frequent occurrence in Phaseolus 
spp., and which was called a ‘ Zwischenstrang’ by Dodel i 1 these structures 
are often present in Helianthus also, and usually take their origin at a 
higher or lower level in the hypocotyl — frequently in the collet region. 
Syncotyl B. In this seedling the cotyledons were fused together for 
about half their length (Fig. 32). The root was found to be triarch in the 
main, though towards the apex it became practically diarch by the disap- 
pearance of the protoxylem in the intercotyledonary plane. The triarch star 
became medullated in the usual way below the collet, and the xylem-ring 
opened out into the C-shape recorded for dicotyls, but with only three 
protoxylems : two of the phloems also dividing radially so as to produce 
five groups (Fig. 13). Just below the collet the xylem-ring broke into 
pieces as indicated by the dotted lines in Fig. 13, with the production 
of two ordinary triads (or double bundles) and a single asymmetrical 
lateral cotyledon trace (Fig. 14). That is to say, we have the dicotylous 
structure with the omission of one of the lateral cotyledon traces, viz. the 
one on the side of the axis on which the cotyledons had united. The 
condition of Fig. 15 was reached about % mm. above the collet and persisted 
through the greater part of the hypocotyl. In the upper part of the axis 
plumular bundles appeared, but no ‘ Zwischenstrange 5 were present in this 
seedling (Fig. 16). The single lateral cotyledon trace behaved in the same 
way, dividing between the two edges of the seed-leaf (Fig. 17). 
Thus the first marked effect of syncotyly on anatomy is simply the 
elimination of one of the two lateral cotyledon traces, and the corresponding 
reduction of the root stele from tetrarchy to triarchy. 
Syncotyl C. The cotyledons were fused for about three-quarters of 
their whole length. The root was triarch and the general structure was 
very similar to that of Syncotyl B, except that through the greater part of 
the hypocotyl the adjacent metaxylems of the two double bundles were 
almost in contact with one another, their phloem groups not separating 
until just below the cotyledonary node. A further difference was the 
presence of an accessory cotyledon strand (cf. Syncotyl A). 
Syncotyl D. The cotyledons were united to the tip, there being 
a scarcely perceptible sinus to mark the line of fusion. This seedling 
carried further the modification foreshadowed in Syncotyl C. The two 
adjacent metaxylems of the triads lay practically in contact from the root to 
some distance up the cotyledon lamina, and the superjacent phloem group 
remained entire throughout the axis (Fig. 18). The lateral cotyledon trace 
on the side away from the fusion occurred and behaved as usual. 
Advanced syncotyly thus results in considerable compression of the 
vascular system towards the line of fusion. 
1 Dodel ( 72 ), p. 156 ; Compton (’ 12 b ), p. 61. 
