5*8 
Holden and Bexon.—Observations on the 
the seedlings of this second group the cotyledon is split almost to the base. 
In the seedlings of the first group the epicotyledonary leaves, when present, 
are invariably two in number; but in one seedling of the second type there 
are three epicotyledonary leaves. 
We propose to term those hemitricotyls which show single collateral 
bundles in the abnormal member ‘ Type a ’ and those which have double 
bundles * Type /3 \ 
Tricotyls. 
The thirty-two tricotylous seedlings examined fall, with one exception, 
into a series which is readily derivable from the type (3 hemitricotyl. The 
earliest stage is one in which the triarch stage produced by the passage of the 
three double bundles into the hypocotyl is extremely transitory, one of the 
Fig. io. a-f. Tricotyl showing reduction from triarchy to diarchy by fusion. Note the assumption 
of ‘ T ’ shape followed by ‘ Y ’ shape in the xylem during transition. 
phloem groups becoming smaller and rapidly dying out, this being followed 
by the fusion of the xylem arms on either side of it. The fusion of the 
xylem arms to produce diarchy is in some cases preceded by the assump¬ 
tion of a ‘ T ’-shape followed by a ‘ Y ’, in which the oblique arms are very 
short (Fig. io); in other cases the ‘ Y ’-shape is assumed directly without 
the intervention of a ‘ T ’ stage (Figs. 8 and 9). 
From the type in which diarchy is rapidly developed, a series of stages 
is found in which diarchy is produced at a later and later stage, until it 
appears only in the apical region of the root, the hypocotyl and the major 
portion of the root showing a triarch structure. In other cases the apex of 
the root shows a ‘ Y ’ structure with two phloem groups, whilst others again 
show a distinctly unequal development of the xylem plate, two of the arms 
being less robust than the remaining one, and showing a relatively small 
phloem group between them. There is little doubt that if these seedlings 
had been allowed to develop further, the apical portion of the root would 
