295 



CHAPTER XXIV 



THE DISPERSAL AND GEOGRAPHICAL 

 DISTRIBUTION OF WATER PLANTS 



THE most striking character of the geographical dis- 

 tribution of water plants is, in general, their remark- 

 ably wide range 1 . Countless instances might be cited, but it 

 may perhaps suffice to refer, as examples, to Potamogeton crispus y 

 which occurs in Europe, Asia, Africa, America and Australia, 

 and to Ceratophyllum demersum and Lemna minor, which are also 

 found almost all over the world. Of the twenty-two genera of 

 Lythraceae, again, only five are common to both hemispheres 

 Rotala, Ammania^ Pep/is, Lythrum and Nesaea and these 

 five all characteristically frequent water or marshy ground 2 . The 

 wide distributions of aquatics often include occurrences on 

 islands which are some distance from other land surfaces; Lemna 

 trisuha, for example, which is found in Europe, Asia, North 

 and South America, Australia and Africa, penetrates to Mauri- 

 tius, Madeira, the Azores and the Canary Islands. The most 

 marked exception to the rule of the wide distribution of hydro- 

 phytes is furnished by the Podostemads 3 , many of which inhabit 

 extremely restricted areas. The Brazilian river Araguay, for 

 instance, has three sets of cataracts, each of which is populated 

 by an almost entirely different group of species belonging to 

 this family. Seven species of Castelnavia occur in this river, 

 although the genus is almost unknown elsewhere. 



If we except the Podostemads, the generalisation certainly 

 holds good that aquatic Angiosperms have, as a rule, a wider 

 distribution than the terrestrial members of the group. This 



1 Schenck, H. (1885), gives the ranges of a long series of aquatic 

 plants, as far as they were known at that date. 



2 Gin, A. (1909). s Weddell, H. A. (1872). 



