CHAPTER XIV 



ISOETACE^ 



The genus Isoetes, the sole representative of the family Isoe- 

 tacese, differs so much from the other Pteridophytes that there 

 has been a good deal of difference of opinion as to where it 

 should be placed. Isoetes is most commonly associated with 

 Selaginella, and there are undoubtedly marked resemblances be- 

 tween the two genera in certain anatomical details, and in the 

 development of the spores and gametophyte. On the other 

 hand, the embryo and the spermatozoids are much more like 

 those of the lower Ferns, with which they have sometimes been 

 associated. Whether the Isoetacese are assigned to the Fili- 

 cineae or Lycopodinese, they are sufficiently distinct to warrant 

 the establishment of a separate order, Isoetales. 



According to Sadebeck (8), there are 62 species of Isoetes. 

 Of these sixteen are found in the United States. 



Isoetes has been the subject of repeated investigation, Hof- 

 meister ( i ) being the first to study its development in detail. 

 The sporophyte is in most species either aquatic or amphibious, 

 but a few species are terrestrial. They are very much alike in 

 appearance, having a very short stem whose upper part is com- 

 pletely covered with the overlapping broad bases of the leaves, 

 which themselves are long and rush-like, so that the plant in 

 general appearance might be readily taken for an aquatic 

 Monocotyledon. The roots are numerous and dichotomously 

 branched. The stem grows slowly in diameter, and the older 

 ones show two or three vertical furrows that unite below, and 

 as the stem continues to grow these furrows deepen, so that the 

 old stem is strongly two or three lobed. In the furrows the 

 roots are formed in acropetal succession. The leaves are closely 

 set and expanded at the base (Fig. 309) into a broad sheath, 



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