138 



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 

 Isoetis (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 vsiscular 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 

 be associated with that group. 



Leaves. — The leaves are unique in 

 structure, being arranged in a close 



