ENGELMANN — THE GENUS ISOETES IN N. AMERICA, 373 



usually emerged : /. saccharata, I. riparia, I. flaccida, I. Engebnanni, 

 I. melanospom^ and probably /. Cubana, 



c. Terrestrial species, which grow during the greater part of 

 their life on dry (or moist) land : /. melanopoda^ I. Butlefi, and /. 

 Ahittal/ii. 



These divisions seemed to be the most natural ones so long as 

 but few species were known, and when other characters, such as 

 the presence or absence of stomata, seemed to correspond with 

 and to confirm them ; but, since we know more species and more 

 about them, we have learned that structural and biological char- 

 acters do not always go together ; and, moreover, that some 

 species are in some forms submerged and in others almost am- 

 phibious (/. echinospora)^ and that some amphibious ones become 

 sometimes almost terrestrial (/. E?igelmafini). A less important 

 objection is, that the biological characters cannot be made out in 

 herbarium specimens. 



3. According to the condition of the velum : 



a. A partial or narrow velum has /. lac us tr is, I.pygmcBa, I. Tuck- 

 erimni^ I. echinospora^ I. saccharata, I. Bolanderi^ I. riparia, 1, En- 

 gebnanni^ I. Howellii, I. niela?wpoda, and /. Cuba?ia. In /. Butleri 

 the velum is almost completely absent. 



b. A complete velum has I. melanopoda, I.flaccida, and /. Niit- 

 tallii. 



The anatomical structure of the leaves, viz., the presence or 

 absence of stomata and of peripheral bast-bundles, furnishes us 

 valuable characters, and, though somewhat difficult to ascertain 

 and therefore less practical, may after all have to be placed in the 

 first line, as it seems to correspond best with the essential charac- 

 ters of the plant. 



4. According to the presence or absence of stomata in the 

 leaves : 



a. Without any stomata are only /. lacustris, I. pygmcea, and /. 

 Tuckermani, The typical European /. echinospora also belongs 

 here, but the different American forms of this species must be 

 classed among the next, though some of them have only very few 

 stomata, often difficult to make out. 



b. With stomata (generally abundant) : / echinospora (Ameri- 

 can varieties), /. saccharata^ I. Bola7ide7i, I. riparia, I. melanospora, 

 I. Engebnanni, I. Howellii^ I. Jiaccida, I. nielanopoda^ I. Butleri^ 

 I. Nnttallii, and /. Cubana. 



