60 Sargant . — Theory of the Origin of Monocotyledons 
cut through a second-year tuber by Miss Thomas shows that 
no regular network exists there. The xylem strands are 
better developed than in the first-year tuber, but travel down- 
wards in the same isolated way. Where a cauline root is 
given off, the xylem strands in the immediate neighbourhood 
collect together and anastomose. 
The irregularities in the vascular system of first-year tubers 
affect the four xylem strands only which are in the neighbour- 
hood of phloem groups, and do not extend to the two pairs of 
slender xylem strands between them (x mi x m , in Fig. 5, PI. VI). 
Their behaviour is remarkably uniform in all three seedlings. 
Each pair is derived, as has been shown, from a single xylem 
strand, and the two halves reunite about halfway down the 
tuber, or even earlier. The two xylem groups thus formed on 
opposite sides of the tuber preserve their identity, and are 
continued downwards into the root, where they form the two 
permanent protoxylem groups of the root-stele. It is exceed- 
ingly rare to find any connexion between these xylem groups 
and the other strands scattered round the tuber. 
These scattered strands draw together in two groups, 
phloem and xylem alike, and form two bundles facing each 
other. Fig. 6 on PI. VI shows the orientation of such 
a stele in process of formation. To right and left are clusters 
of xylem elements unaccompanied by phloem, and with the 
protoxylem elements already external. To the north-east of 
the stele, as it were, is a slender bundle having an external 
phloem group, three lignified elements within it representing 
the primary xylem, and two radial rows of two unligni- 
fied elements each to represent the secondary formation. 
A similar group is in course of formation on the opposite 
side. 
At the base of the tuber the stele closes round the centre 
of the section, and a tetrarch xylem plate is formed. Two 
phloem groups only are present, and these are placed outside 
the two lateral protoxylem rays (px\ px', in Figs. 1 and 3 on 
PI. VII). The outermost elements of these rays can be 
traced backwards and identified as part of the primary forma- 
