[Vol. 9 

 92 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



more dry seasons. Still others grow where alternately submerged 

 and exposed during each growing season. 



Ecologically, therefore, it is convenient to subdivide the group 

 into (1) submersed, (2) amphibious, and (3) terrestrial forms, 

 though this does not represent systematic relations. 



The geographic range of the individual species is usually lim- 

 ited, but representatives of the genus are found in practically 

 all explored countries. In Europe, France leads with five species ; 

 in Africa, Algeria with four species and one variety. In North 

 America, Massachusetts with five species has probably more re- 

 corded stations than any other state. California has six species 

 recorded, one of which is truly terrestrial. At present, it is 

 difficult to determine centers of distribution, but the accumula- 

 tion of more data should help in ascertaining relations. 



By means of a tabular survey of some North American species, 

 one can gain an idea of the range of species relations in the matter 

 of habitat, plant associates, structural diversity, etc. In every 

 subdivision, one can find examples from different regions of the 

 world. North American forms, particularly those of a somewhat 

 extended range, are chosen simply because of more available 

 data. 



It may be noticed that all submersed or amphibious forms 

 are related to fresh water. The original account of a salt-water 

 relation for 7. maritima has since been corrected. The ter- 

 restrial forms, I. melanopoda and 7. Butleri, are found in so-called 

 "alkali flats" where the content of magnesium and sodium sul- 

 phates is high. These flats are very level, and so become very 

 wet in the spring but are very dry in summer, for the clay or hard 

 pan is covered by too thin a soil layer to hold moisture (E. Hall). 



In other regards there is much diversity in habitat relations. 

 Most of the submersed forms are related to lakes or ponds, vary- 

 ing from a few inches to six or more feet in depth. Of the am- 

 phibious forms, some are riparian, notably 7. riparia and 7. 

 saccharata, but most are found in pools or ponds, where they 

 form a marginal zone, thus becoming exposed in thd usual 

 seasonal lowering of the water level. These ponds or lakes may 

 be in meadows, woodland, or as in the case of 7. melanospora, 

 may be merely low depressions in granite rock, which contain a 

 few inches of soil and which in summer retain no water, leaving 

 the plants to be parched and dried by full exposure to the sun. 



