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TRANS. ST. LOUIS ACAD. SCIENCE. 



unusually dry seasons. The majority are of amphibious growth, 

 entirely or partially submerged during 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 

 flat grounds which may be overflowed in spring, or in springy 

 localities, but they fully develope and become fertile only when 

 out of water. /. tnelanopoda, 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 wdiich 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 mud, others between gravel and stones ; the 

 amphibious ones generally in deep mud. Our terrestrial species 

 grow mostly in heavy, retentive soil. 



§ 4. Systematic Arrmigement. 

 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 biiobed^ 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 : /. laciistris, I. pygmcea^ I. Tuckermaiii, I. echinospora^ and /. 

 Bolanderi. 



b. Amphibious species,^\\\z\\ grow in water but have their leaves 



