298 DISPERSAL [CH. 



possesses a fruit which sinks like a stone, and the plant is soon 

 killed by sea water yet it has established itself nearly all over 

 the globe, reaching such islands as the Bermudas and Fijis. The 

 Potamogetons, again, present little or no obvious capacity for 

 dispersal by sea yet such a species as Potamogeton densus^ 

 whose fruits sink at once in fresh or salt water, flourishes in 

 Europe, Asia, Africa and America. 



Water plants, as we have already pointed out, are particu- 

 larly prone to reproduction by vegetative means, and any theory 

 attempting to account for their dispersal must take into con- 

 sideration the conveyance of detached fragments, and of various 

 types of winter-buds or turions, which are probably more 

 effective than fruits and seeds in the process of dissemination. 



The hypothesis has been proposed that water-fowl are the 

 ' chief agents in the dispersal of hydrophytes. This theory cer- 

 tainly explains a large proportion of the observed facts, and a 

 considerable amount of indirect and circumstantial evidence 

 has accumulated in its favour. Darwin 1 pointed out how readily 

 wading birds, which are great wanderers, might convey seeds 

 from one water basin to another, in the mud adhering to their 

 feet. Clement Reid 2 came to conclusions bearing on this ques- 

 tion in the course of his study of the colonisation of isolated 

 ponds such as pools which collect in old brick yards, quarries, 

 etc., and the dew ponds dug on dry chalk downs to provide 

 water for cattle and sheep. He found, in general, that the water 

 plants which colonise isolated ponds are essentially the floating 

 species with finely divided leaves. Their seeds and fruits are 

 commonly such as would be digested and destroyed if eaten by 

 birds, but their stems are brittle, and their leaves, on removal 

 from the water, collapse and cling closely to any object they may 

 touch. He therefore concluded that it was probable that these 

 plants are transported in fragments that adhere to the feet of 

 wading birds. This would also account for the constant presence 

 of Limnaeids in these ponds, since their eggs might easily be 

 carried, clinging to pieces of leaves or stems. 



1 Darwin, C. (1859). 2 Reid, C. (1892). 



