440 Scott and Hill.—Structure of Isoetes Hystrix . 
xylem. At this point the primitive tracheides of the root- 
bundle are extremely short, showing the characters of those 
of the stele. 
Prof. Bertrand (i88i,p. 51) figures a root of Isoetes Hystrix 
in transverse section, and describes the roots of Isoetes as 
having a bicentric bundle, 5 curved so strongly that some 
authors have considered these bundles as having only a single 
centre of development.’ His figure clearly shows two proto- 
xylem-groups side by side. We have seen no such structure 
in the roots of Isoetes , except at the point of bifurcation ; in 
every case the protoxylem is first differentiated as a single 
tracheide. At later stages, where several tracheides have 
become lignified, it might be possible to mistake some of 
the later-formed elements for a second group of protoxylem. 
Probably, however, Prof. Bertrand’s interpretation is to be 
explained in a different way. His figure exactly represents the 
structure of an Isoetes root, at the point of bifurcation , the two 
protoxylem-groups belonging to the two forks of the dicho¬ 
tomizing bundle. Our Fig. 34 unay be compared with 
Prof. Bertrand’s, but ours was drawn from a section where the 
dichotomy was not quite as far advanced as it appears in his. 
The whole of the evidence, so far at least as /. Hystrix is 
concerned, appears to us to leave no doubt that the structure 
of the root-stele is in all parts a strictly monarch one, and 
that there is at present no indication that this monarch 
organization has arisen by reduction from any more complex 
structure. 
Summary. 
I. The Stem. 
1. There is some evidence that the apex may grow by 
means of a single apical cell, as Hofmeister supposed. 
2. The stele is not composed of the united leaf-traces, but 
is best interpreted as a cauline structure, comparable to that 
of the simpler Lycopods. 
3. The differentiation of the primary wood is nearly simul- 
