1 6 ENGELMANN THE GENUS ISOETES IN N. AMERICA. [ 37 $ 
usually emerged : Z saccharata, Z riparia, Z flaccida, I. Engelmanni 
I. melanospora , and probably /. Cubana. 
c. Terrestrial species, which grow during the greater part of 
their life on dry (or moist) land: I. melanopoda , I. Butleri, and I. 
Nuttallii. 
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 (Z Engelmanni). 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 I. lacustris, I.pygmcea , I. Tuck- 
ermani , I. echinospora , Z. saccharata , I. Bolanderi , I. riparia, 1 . En¬ 
gelmanni, I. Howellii , Z. melanopoda, and I. Cubana. In /. Butleri 
the velum is almost completely absent. 
< 5 . A complete velum has Z melanopoda, I. flaccida, and Z Az//- 
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 I. lacustris, Z pygmcea, and Z. 
Tuckermani. The typical European Z 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) : Z echinospora (Ameri¬ 
can varieties), Z saccharata , Z. Bolanderi, I. riparia, I. melanospora , 
Z. Engelmanni, I Howellii , Z flaccida, I. melanopoda , Z. Butleri, 
Z Nuttallii, and I. Cubana . 
^OlkL 
