THE GENUS ISOETES IN NORTH AMERICA. 459 
§ 4. Systematic ARRANGEMENT. 
Our 14 species can be classed, — 
1. According to the development of the trunk: thus we have 13 species, all our North Ameri- 
can ones with a bilobed, and only the single Cuban one with a trilobed trunk. 
2. According to their mode of growth: — 
a. Submerged species, which normally always grow under water: J. lacustris, I. pygmea, I. Tuck- 
ermant, I. echinospora, and I. Bolanderit. 
b. Amphibious species, which grow in water but have their leaves usually emerged: [373 (16) ] 
I. saccharata, I. riparia, I. flaccida, I. Engelmanni, I. melanospora, and probably J. 
Cubana. 
ce. Terrestrial species, which grow during the greater part of their life on dry (or moist) land: J. 
melanopoda, I. Butleri, and I. Nuttallit. 
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 struc- 
tural and biological characters do not always go together; and, moreover, that some species are in 
some forms submerged and in others almost amphibious (J. echinospora), and that some amphibious 
ones become sometimes almost terrestrial (J. 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 in J. lacustris, I. pygmea, I. Tuckermani, I. echinospora, I. sac- 
charata, I. Bolanderi, I. riparia, I. Engelmanni, I. Howellii, I. melanopoda, and I. Cubana. In J. 
Butleri the velum is almost completely absent. 
b. A complete velum in L. melanopoda, I. flaccida, and I. Nuttallit. 
The anatomical structure of the leaves, viz. the presence or absence of stomata and of periphe- 
ral 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 characters of the plant. 
4. According to the presence or absence of stomata on the leaves : — 
a. Without any stomata are only LI lacustris, I. pygmea, and I. Tuckermani. The typical Euro- 
pean JZ. 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): J. echinospora (American varieties), J. saccharata, I. Bo- 
landeri, I. riparia, I. melanospora, I. Engelmanni, I. Howellii, I. flaccida, I. melanopoda, I. Butlert, 
I. Nuttallii, and I. Cubana. 
5. According to the presence or absence of peripheral bast-bundles : — ate (17)] 
a. Without bast-bundles: J. lacustris, I. pygmea, I. Tuckermani, I. echinospora, 
saccharata, I. Bolanderi, I. riparia, and I. melanospora. 
b. With bast-bundles: Z. Hngelmanni, I. Howellii, I. flaccida, I. melanopoda, I. Butleri, I. Nut- 
tallai, and I, Cubana. 
The comparative size or the sculpture of the spores, and the number and length of the leaves, 
furnish no legitimate grounds for classification ; still it may be mentioned here, that the largest 
macrospores (0.45 to 0.80 mm. in diam.) are found in J. lacustris, I. Butleri, and J. riparia ; the 
smallest (0.25 to 0.45 mm.) in J. melanopoda, I. flaccida, and L. Engelmanni, var. valida. 
The largest number of leaves, 50 to 200, are observed in the last-mentioned form, in J. melano- 
poda, and in J. Nuttall; the smallest number, 5 to 20, in L. pygmea, I. melanospora, I. saccharata, 
and J. Bolanderi 
