58 Sargant. — Theory of the Origin of Monocotyledons 
of tuber-formation in this species is thus quite distinct from 
that described in Delphinium sp. and Anemone coronaria. 
The increase of girth was there due to the formation of 
secondary tissue by a normal cambium, and the plumular 
traces took a large share in the formation of its vascular 
system. 
The behaviour of the cotyledonary traces when they enter 
the axis is indicated in Diagram X and very fully illustrated 
on Plates VI and VII. 
The sections drawn in Figs. 2-6 of PI. VI and Figs. 1 and 2 
of PI. VII are from a single series cut through the tuber of 
A 
B 
m o*©*o Vi 2 
( « » 
\ O'* ^0 J 
Diagram X. 
a seedling younger than that outlined in Fig. 1, PI. VI. The 
bundles of the cotyledon show traces of double structure even 
before they leave the cotyledonary tube. There are traces 
of cambium between the phloem and xylem of each (PI. 
VI, Fig. 3). The compound nature of the traces becomes 
more clearly evident as they enter the tuber. A little 
lower down the phloem groups of each pair have drawn 
further apart, and the xylem elements are in three clusters : 
one internal to either group of phloem and one solitary 
between the other two (Fig. 4). This intermediate cluster 
then breaks up into two parts, and we have eight xylem and 
four phloem groups arranged symmetrically in two parallel 
straight lines, each of which is equidistant from the periphery 
of the section and its centre (Fig. 5). 
Down to this level the three complete series of sections 
