Adaptation in the Tristichacece and Podostemacece. 



53'5 



confined to the tropics and sub-tropics, and the coldest water in which I 

 have ever seen any was 57° F. (14° C), in the Khasia Hills of Assam, the 

 warmest 81° F. (27° C). Their temperature conditions are thus more 

 uniform than those of other water plants. Their conditions of light 

 supply are also uniform. They live only from the water-level down 

 to perhaps 50-75 cm. The medium in which they live, the running 

 water of tropical rivers, is very uniform, and their food supply also, as 

 it comes from the passing water. The only differences between them 

 in the numerous localities in which they occur are therefore in the speed of 

 the water current. The fastest water in which I have found any was in the 

 Eio Piabanha, north of Petropolis, near to Eio de Janeiro, where Lophogyne 

 arculifera was growing in water going from '2\ to 4 miles an hour, and the 

 slowest was at Hakinda, near Peradeniya, in Ceylon, where Farmeria 

 metzgerioides and Podostemon subulatus were growing in an eddy that moved 

 perhaps half a mile in the hour. But, if this involved any difference in 

 structure, there would surely be more " Zugfestigkeit " in the thalli of the 

 more rapid water, and an examination of the various sections given in my 

 monograph of the Indian forms will show that this is not so — some have it, 

 some not. 



It is true that at any one actual spot, of, say, a square yard, on a rock, 

 there are most commonly only one or two species to be found growing, but 

 this does not mean that no others could grow there, but simply that each 

 one has some slight preference in positions. Always there is more or less of 

 intermixture. For instance, from my Indian monograph I may quote the 

 various species, with those which were recorded as found together with them : — 



Tristicha ramosissima : Griffithella Hookeriana. 



Lawia zeylanica : Hydrdbryum olivaceum, lichenoides, Farmeria metzgerioides, 

 Podostemon subulatus, Dicraza stylosa, Griffithella Hookeriana. 



Podostemon subulatus : Dicrcca elongata, stylosa, Farmeria metzgerioides, 

 Latvia zeylanica, Hydrdbryum olivaceum. 



Podostemon Barberi : Griffithella Hookeriana. 



Dicraza elongata : Podostemon subulatus, Dicrcea stylosa, Hydrdbryum 

 olivaceum. 



Dicraza dichotorna : Hydrdbryum olivaceum. 

 Dicroza Wallichii : Hydrdbryum lichenoides. 



Dicraza stylosa : D. elongata, Podostemon subulatus, Hydrdbryum olivaceum, 

 lichenoides, Lawia zeylanica, Griffithella Hookeriana. 



Griffithella Hookeriana : Lawia zeylanica, Podostemon Barberi, Dicrcece 

 stylosa, Tristicha ramosissima. 



Willisia sela/jinoidcs : Hydrdbryum lichenoides. 



i 



