82 Bexon . — Observations on the Anatomy of 
a cotyledonary tube, but this is never pronounced and is entirely absent in 
many instances. Each cotyledon at its base contains a median collateral 
vascular bundle and two laterals, these, however, fusing with the central 
bundle before the cotyledonary node is reached. ‘ Rotation * commences 
usually in the base of the cotyledon, although it has been observed in some 
cases to be delayed until the apex of the hypocotyl is reached. The 
phloem mass of the median collateral bundles divides into two portions 
which separate slightly, whilst the protoxylem commences a gradual move- 
ment outwards, being first mesarch and later exarch in position. This is 
accompanied by a slight flattening of the vascular bundle, without however 
any wide separation of the metaxylem into two groups such as gives the 
characteristic ‘ double-bundle ’ appearance in Cheiranthus and many other 
forms. ‘ Rotation ’ is completed soon after the entrance of the bundles into 
the hypocotyl, but for a short distance the centre of the hypocotyl is 
occupied by a fairly large pith. This decreases in size and finally dis- 
appears, so that midway down the hypocotyl a solid diarch plate of xylem 
is produced, flanked by two groups of phloem. 
Hemitricotyls . The hemitricotylous material examined included all 
possible stages of cotyledonary fission from the one extreme in which the 
lamina of the abnormal cotyledon showed no sign of splitting, this being 
indicated merely by the forking of the central vascular bundle, to the other 
in which the seedling was scarcely distinguishable from a tricotyl (Fig. i). 
As regards anatomical structure the hemitricotyls were divisible into 
three distinct groups according to the behaviour of their vascular bundles. 
The first group, which included more than half the total number of 
seedlings examined, corresponded to the group which in the investigation of 
Cheiranthus Cheiri (9) was styled type a. In these seedlings each lobe 
of the bifurcated cotyledon was supplied by a collateral vascular bundle, 
the two bundles gradually approaching one another and finally fusing to 
form a single bundle, which in the transition region formed one pole of the 
diarch root (Fig. 2). The level at which the fusion of the bundles occurred 
showed great variation, and a series of forms was obtained in the simplest 
members of which the level of fusion was fairly high in the lamina, whilst in 
others it occurred in the upper and lower portions of the cotyledon petiole, 
and in the extreme cases at the apex of the hypocotyl. 
It is noteworthy, however, that the level of fusion of the vascular 
bundles bore no relation to the extent of the split in the cotyledon, for in 
three seedlings in which the fission extended for one-third, one-half, and 
three-quarters of the length of the cotyledon respectively, the vascular 
bundles in all cases fused in the base of the petiole, whilst an even more 
extreme instance was found in two seedlings in which, while the lamina 
showed no sign of fission, the bundles did not fuse until the point of entry 
into the hypocotyl. 
