64 Sargant.- — Theory of the Origin of Monocotyledons 
downwards (Fig. 4, PL VII). The length of the hypocotyl 
defined by its external characters varies from 5 mm. to 1 mm., 
or even less in the seedlings I have examined. 
The external features of this seedling have been so fully 
described by Irmisch ( 21 , p. 1) and Sterckx ( 1 . c.) that I may 
go on at once to its vascular structure. The three seedlings 
A 3 , A 4 , and B 1? from which I have cut complete series of 
sections, agree with each other in every detail. That figured 
on PL VII is the youngest of the three. 
A single massive bundle runs down the whole length of 
the cotyledonary petiole. It is enclosed in a well-defined 
endodermis and contains a normal cambium layer. The 
phloem is a compact rounded strand, and there is a single 
protoxylem group, but the elements of the metaxylem form 
two distinct clusters separated by a few thin-walled cells. At 
the base of the cotyledon the petiole is bordered by two 
membranous wings which are united round the embryonic 
bud of the plumule into a closed sheath. At this level the 
cotyledonary trace has opened out slightly : the phloem mass 
is divided as well as the metaxylem. A single plumular trace 
joins it at the first node (Figs. 6 and 7). As they meet, the 
plumular trace becomes double and opens out in the same 
way as that from the cotyledon (Fig. 7). Very little below 
this both groups of protoxylem have become external, and 
a diarch root is constituted (Fig. 8). The process of transition 
recalls that found in Zygadenus. 
The length of the hypocotyl defined by its vascular 
characters does not exceed *5 mm. in any of the seedlings cut. 
The lateral position of the cotyledonary member is clearly 
an advantage to the plant. It allows the foliage leaves to 
develop unchecked by the necessity of bursting through a 
tubular sheath, as they must do in Delphinium nudicaule and 
Anemone coronaria. The ‘ cotyledon ’ of Ranunculus Ficaria 
may possibly have been derived from a tubular fusion such 
as that found in these plants. R.parnassifolius possesses a long 
cotyledonary tube (Winkler, 44 ), and R. millefoliatus a shorter 
one (Irmisch, 26 , p. 29). But it seems more likely that the 
