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TRANS. ST. LOUIS ACAD. SCIENCE. 
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unusually dry seasons. The majority are of amphibious growth, 
entirely or partially submerged dnring the wet seasons, in win. 
ter and especially in early spring ; but at the growing season they 
partially get out of the water, leaving only their trunk and lower 
part of the leaves immersed. These species do not come to per¬ 
fection when completely immersed, though they may not be en¬ 
tirely infertile ; it seems that partial exposure to the atmosphere 
is necessary to their well-being. A variety of the amphibious 
species are the tidal ones, which are alternately emerged and 
submerged during the changes of the tides; they are found in - 
the estuaries of some of our Atlantic rivers. Then we have a 
few species which we are justified in calling terrestrial, as we 
find them, when fully developed, on dry land ; but it seems that 
for their germination and their growth in early spring they also 
require moisture and water; we therefore find them on low or 
fiat grounds which may be overflowed in spring, or in springy 
localities, but they fully develope and become fertile only w;hen 
out of water. I. melanopoda, which belongs to this class, normally 
matures in June or early*in July, and its leaves wither before the 
end of that month ; by keeping it immersed, however, I have 
kept it fresh and growing throughout the summer, but under 
these conditions it remained perfectly sterile. Those terrestrial 
species which grow on arid hills around the Mediterranean must 
require even less moisture, and are probably satisfied with tem¬ 
porary drenchings. 
The submerged and the amphibious species are generally 
found, some in soft rttud, others between gravel and stones; the 
amphibious ones generally in deep mud. Our terrestrial species 
grow mostly in heavy, retentive soil. 
§ Jf.. Systematic Arrangement . 
Our 14 species can be classed—- 
1. According to the developement of the trunk ; thus we have 
13 species, all our North American 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 wa¬ 
ter : /. lacustris, 1 . pygmcea , /. Tuckermani , /. echinospora , and I. 
Bolanderi . 
b. Amphibious species, which grow in water but have their leaves 
