CH. xvii] ELODEA IN BRITAIN 211 



luxuriant at Burton-on-Trent where it had been recorded in 

 1 849^ that it bid fair to block up one of the two streams into 

 which the Trent there divides. Unfortunately the Curator of 

 the Cambridge Botanic Garden, who had received the plant 

 from Professor Babington in 1847, introduced it into a tribu- 

 tary of the Cam in 1848. By 1852 it had spread into the river, 

 and so completely choked it as to raise the water level several 

 inches, and to prevent fishing, swimming and rowing, and 

 greatly to hinder the towing of barges. At this date it first 

 invaded the fen district, and in a few years so choked the dykes 

 as seriously to impede drainage. The difficulties caused by the 

 presence of excessive quantities of the plant were so acute that 

 an adviser was sent down bv the Government to consider the 

 best method of dealing with the pest. No successful plan for 

 coping with it was discovered, but in a few years the luxuriance 

 of the Elodea diminished without any apparent cause. Siddall'^, 

 to whom we owe the most exhaustive treatment of the subject, 

 concludes that, "The experience of those who have had most 

 to do with it seems to indicate that if left alone, its habit is, 

 upon first introduction into a new locality, to spread with alarm- 

 ing rapidity; so much so as literally to choke other water plants 

 out of existence. But this active phase reaches a maximum in 

 from five to seven years, and then gradually declines, until at 

 last the Anacharis \Elodea\ ceases to be a pest, and becomes an 

 ordinary denizen of the pond, river, or canal, as the case maybe." 



As has been already stated in Chapter iv, Elodea canadensis 

 never reproduces itself sexually in this country, and the history 

 of the plant suggests that possibly the whole Elodea population 

 of England may be regarded, in one sense, as a single individual, 

 with an enormous vegetative output, mechanically sub-divided 

 into vast numbers of apparently distinct plants; in other words, 

 it is not improbable that it may represent the soma developed 

 from a single fertilised ovum. It would thus be a "major plant 

 unit," whose soma consists of a vast number of minor indivi- 

 duals. Pallis"^, in a most suggestive study of the problem of 



1 Caspary,R. (18582). 2 SiddaliJ. D. (1885). Pallis,M. (1916). 



14 2 



