520 
Holden and Bexon.—Observations on the 
alternating with the cotyledons, but in one case only two epicotyledonary 
leaves were present. It is interesting to note also that in some instances 
the vascular bundles of the epicotyledonary leaves showed the ‘ doubleness * 
which is so characteristic of the cotyledonary strands. 
The exceptional case referred to at the beginning of this section is 
derivable from a type a hemitricotyl. In it two of the cotyledons possessed 
a collateral bundle, whilst the third showed a normal double bundle (Fig. 
12). As they enter the hypocotyl the two collateral bundles are a little 
distance apart, but they commence to rotate and approach one another until 
the typical structure is produced, this occurring, however, at a rather lower 
level than in the case of the double bundle. Ultimately a diarch plate is 
formed. There were two epicotyledonary leaves present, these being 
situated on either side of the normal cotyledon. 
Hemitetracotyl. 
The single hemitetracotyl examined was essentially similar to the 
type a hemitricotyls, the two normal cotyledons having each a double 
bundle, whilst the midrib of each half of the bifurcated cotyledon was 
formed by a collateral strand. The two collateral strands behaved in tran¬ 
sition as the constituent halves of a double bundle, so that triarchy was 
established immediately, and was maintained throughout the root. 
Tetracotyls. 
The twelve seedlings examined showed considerable variety in their 
vascular arrangements, but the majority formed a similar series to the 
tricotyls. In the simplest case the cotyledonary bundles, which are grouped 
in two obvious pairs, show a transient tetrarch arrangement at the top of 
the hypocotyl, this being followed by a reduction to diarchy without the 
intervention of a triarch stage, owing to the bundles fusing in pairs. This 
fusion is accompanied by the disappearance of the phloem group originally 
lying between each pair of xylem masses (Fig. 14). 
The second type is one which leads up to stable tetrarchy and consists 
of the initiation of a tetrarch stage at the top of the hypocotyl, followed 
sooner or later by a reduction to the triarch condition (Fig. 15). In some 
seedlings the tetrarch stage was very brief, but in others it was retained 
farther and farther down the hypocotyl, until finally the tetrarch arrange¬ 
ment obtained throughout (Fig. 16). 
The reduction from tetrarchy to triarchy is accomplished in two 
different ways, the two methods being almost equally represented among 
the seedlings. In some instances reduction is brought about by the fusion 
of two of the xylem masses, this being preceded by the loss of the phloem 
lying between them (Fig. 15). In the second type of seedling the reduction 
is produced by the gradual dying out of one xylem plate and the fusion of 
