796 
Compton. — An Anatomical Study of 
Thus at the base of the hypocotyl there are constituted six collateral 
bundles of xylem and phloem, and the two small median polar protoxylems 
(Fig. 6). This condition persists to the summit of the hypocotyl, slight 
rearrangements in the xylem taking place with the result that each group 
becomes truly endarch. 
In the upper half of the hypocotyl the plumular procarnbium appears 
in four tangential bands, each lying between a polar triad and one of the 
lateral cotyledon traces. 
There is a short cotyledon tube, at the base of which the cotyledonary 
vascular system which has ascended from the root moves outwards, leaving 
the epicotyledonary procambia behind to pass up into the plumule. The 
two lateral bundles of the hypocotyl divide radially, and on the cotyledons 
separating, each takes half of each bundle as well as the main polar 
triad (Fig. 7). 
The base of each cotyledon therefore contains (1) the triad or ‘double 
bundle 5 (Fig. 8) derived from the whole of a root-xylem ; this comprising 
two collateral bundles of metaxylem and phloem, and a few effete scattered 
protoxylem elements in between — the latter sometimes accompanied by a 
tiny detached group of phloem : 1 (2) two collateral bundles, one at each 
edge of the cotyledon, each derived from half a root xylem. 
The cotyledons are slightly asymmetrical, one side being more curved 
than the other : the venation is shown in Fig. 9. 
Syncotyls. There are many degrees of fusion of cotyledons, ranging 
from seedlings whose cotyledons are united by the petioles alone on one 
side, to those which have apparently a single broad cotyledon with a barely 
perceptible apical notch, but with a double vascular supply. Amphisyncotyls, 
i. e. seedlings whose cotyledons are united by both edges, also occur, but 
more rarely. 
The vascular system varies with the degree of fusion of the cotyledons, 
and a series of cases will be described in order to illustrate the mode of 
variation. 
Syncotyl A. The two cotyledons were only united for about 5 mm., 
and the structure was closely similar to that of a dicotyl. The chief 
differences are illustrated in Fig. 10. There is a slight asymmetry of 
structure at the node, the lateral strand which is on the open side of the 
cotyledon dividing radially earlier than the other. The two edges of the 
seed-leaf have thus a more complex vascular system than the space between 
the two double bundles at the same level (Fig. 11). Another point of dif- 
ference, which, however, has no special connexion with the phenomenon of 
syncotyly, is the presence of an additional collateral bundle in between one 
of the double bundles and a lateral cotyledon trace ; this passes out into 
the cotyledon and divides with the other bundles, forming part of the 
1 Cf. Lupinus spp., Compton ( , 12 b ), pp. 29, 79. 
