THE GREAT GROUPS OF PTERIDOPHYTES 



169 



90. Isoetes. This little group of aquatic plants, known 

 as " quill worts," is very puzzling as to its relationships 

 among Pteridophytes. By some it is put with the Ferns, 

 forming a distinct division of Filicales ; by others it is put 



FIG. 143. A common quillwort (Isoetes lacus- 

 tris), showing cluster of roots dichoto- 

 mously branching, and cluster of leaves 

 each enlarged at base and inclosing a sin- 

 gle sporangium. After SCHENCK. 



FIG. 144. Sperm of Isoetes, show- 

 ing spiral body and seven long 

 cilia arising from the beak. 

 After BELAJEFP. 



with the Club-mosses, and is associated with Selaginella. 

 It resembles a bunch of fine . grass growing in shoal water 

 or in mud, but the leaves enlarge at the base and overlap 

 one another and the very short tuberous stem (Fig. 143). 

 Within each enlarged leaf base a single sporangium is 

 formed, and the cluster contains both megasporangia and 

 microsporangia. The sporangia are eusporangiate, and 

 therefore Isoetes shares with Selaginella the distinction of 



