84 Bexon . — - Observations on the Anatomy of 
phloem groups occupying the plane of bundle symphysis disappeared and 
complete fusion of the xylems followed, the typical diarch structure being 
produced. In other cases, however, a definite triarch structure was 
established at the apex of the hypocotyl, showing three widely separated 
xylem groups, and this condition persisted frequently down to a low level 
in the hypocotyl, or even into the root. 
The reduction from triarchy to diarchy was accomplished in the 
majority of seedlings by the gradual approximation and subsequent fusion 
of two of the xylem arms, this being accompanied by the disappearance of 
the phloem group lying between the fusing arms (Fig. 3). In one case, 
however, after the xylem arms had approached one another quite closely, 
and were almost on the point of fusing, the protoxylem of one xylem mass 
disappeared, so that only the metaxylem took part in the fusion process. 
It must be noted that there was no connexion whatever between the 
extent of fission of the cotyledon and the level at which the reduction to 
diarchy took place, since reduction occurred at the top of the hypocotyl in 
seedlings the abnormal cotyledon of which split in one case only for one- 
third of the length of the lamina, and in another instance almost to the 
base of the petiole, whilst reduction occurred in the root of another seedling 
in which the cotyledon showed only a slight apical cleft. 
A third type of hemitricotyl remains to be described, this being of 
particular interest in that no similar phenomenon was met with in the 
course of the investigation of Cheiranthns seedlings, nor can any record o r 
such structure be found in the literature dealing with polycotyly in Angio- 
sperms. The nearest approach to such a structure is found in the gymno- 
spermous seedlings described by Hill and de Fraine (6), but the corre- 
spondence is not exact. In this type, of which four examples, exhibiting all 
degrees of fission of the cotyledon, have been examined, one of the bundles 
of the abnormal cotyledon appeared somewhat smaller than either its 
fellow or the bundle of the normal cotyledon, and on entering the hypocotyl 
only the two larger bundles rotated, whilst the third retained its collateral 
structure. At a fairly early stage the endarch protoxylem elements of the 
abnormal bundle disappeared, the metaxylem elements alone appearing at 
one side of the diarch plate formed by the two normal bundles (Fig. 4). 
Meanwhile the half-phloem groups arising from the sides of the two normal 
bundles adjacent to the collateral bundle fused with the phloem of the 
latter and produced one long band of phloem. At a later stage there was 
a movement inwards of the metaxylem elements so that they appeared 
almost connected with the diarch plate. In the lower region of the hypo- 
cotyl and in the root a typical diarch structure obtained. 
Tricotyls. Of the thirty-eight tricotylous seedlings examined, only two 
belonged to the type a group in which one of the cotyledons possessed 
collateral bundles, which on entering the hypocotyl rotated towards one 
