MORPHOLOGY 
FIG. 315. Isoetes, showing the 
short unbranched stem bearing di- 
chotomous roots and a tuft of linear 
leaves. 
is embedded in the nutritive region of 
the female gametophyte invested by 
the megaspore coat, and from it there 
extends in one direction, outside of the 
spore coat, an elongating stem bearing 
at its tip a pair of young leaves, be- 
tween which is the stem apex ; and 
in the other direction the elongating 
primary root (fig. 314). 
Isoetes 
General character. The genus 
Isoetes (quillworts) comprises about 
sixty species. It is now usually in- 
cluded among the Lycopodiales, al- 
though in certain important features 
it differs from the other members of 
the group. 
Sporophyte. In general appearance 
Isoetes suggests a tufted grass, growing 
on muddy flats or in the water (fig. 315). 
The stem is very short, unbranched, 
and covered by overlapping leaf bases. 
The vascular anatomy of the stem is 
somewhat confusing, and has been in- 
terpreted variously. The stem is so 
short and the leaves are so numerous 
that the vascular cylinder is little more 
than a vascular plate. It seems to be 
a protostele, however, in which the 
xylem elements have not completely 
filled up their region, and there is no 
recognizable phloem. Such a structure 
is evidently related to that found 
among the Lycopodiales, and there- 
fore in vascular anatomy Isoetes is to 
b e assoc iated with that group. 
r _,, , 
Leaves - ~ The leaves are um q ue ln 
structure, being arranged in a close 
