THE AQUATIC QUILLWORTS. 



SERIES of Isoetes specimens 

 could easily be selected which 

 would show every gradation 

 from the most deeply sub- 

 merged plants to those habit- 

 ually living in dry ground, 

 but such a series would of ne- 

 cessity include specimens of 

 many species. The water-loving species, however, keep 

 close to their own element, and it is only under excep- 

 tional circumstances that they appear above the surface ; 

 while the terrestrial forms are for a great part of the 

 year entirely above water, though growing in moist 

 places. Between these two there are certain species 

 that always live for a part of the year under water 

 and a part of the year exposed, so that for purposes 

 of study it is very convenient to divide the genus into 

 three groups containing aquatic, amphibious, and terres- 

 trial species respectively, though a hard and fast line 

 between them cannot be drawn. The species of the first 

 group have been selected for description in this chapter. 



Braun s ^uillwort. 



If, while floating upon the quiet surface of some small 

 mountain lake, the collector should discern, two or three 



