LINNEAN SOCIETY OF LOXDOX. 7 I 



ABSTRACTS. 

 I. 



The Distribution of Elodea canadensis, Michaux, in the British 

 Isles in 1909. By A. O. Walkek, F.L.S. 



[Read 2nd November, 1911.] 



The history of this plant, so far as our islands are concerned, is 

 fairly well known. The first locality in which it appears to have 

 been recoi'ded, by Mr. John Dew in 1836, was at Waringstown, 

 County Down, Ireland. In 1843 it was reported by Dr. G. 

 Johnston, of Berwick-on-Tweed, in Duuse Loch, Berwickshire. By 

 1850 it had spread to many rivers and reservoirs in Great Britain 

 and become a serious nuisance to navigation and drainage — so 

 much so in Lincohishire, that in 1852 Mr. Eawlinson was sent 

 by the Government to advise as to clearing the dykes in the fens. 

 Attempts to eradicate it by dredging failed, and it was found that 

 the only way of dealing with it successfully was to leave it alone, 

 when it appears to gradually diminish or die out altogether. 



In 1884 Mr. J. D. Siddall, of Chester, published a valuable 

 paper on the structure and history of this plant (Proc. Chester 

 Soc. Nat. Sci., Part iii. 1884, p. 125), from which most of the 

 above information is derived. He states that experience shows, 

 " that if left alone, its habit is, upon first introduction into a new 

 locality, to spread with alarming 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 Anacliaris ceases to be a pest 

 and becomes an ordinary denizen of the pond, river, or canal as 

 the case may be." This maximum period in the neighbourhood of 

 Chester seems to have been between the years 1867 and 1873 ; in 

 1884 Mr. Siddall says it is " far less abundant than formerly," and 

 in April 1909, he wrote that he had some difficulty in finding a 

 piece in a locality where in 1873 all other vegetation was choked 

 out by it. He also says that the circulation of the protoplasm 

 in the leaf-cells was very feeble compared to what it was iu 

 1873 — an important fact, possibly indicating diminishing vitality 

 in the species. The recollection of the writer, who resided in 

 Chester from 1856 to 1889 and remembers the canal there so 

 choked with the weed as to greatly impede the boat traffic, quite 

 bears out Mr. Siddall's statements as to its abundance in 1867 to 

 1873 (l. c. p. 131). 



In 1909 it appeared to the writer that sufficient time had 

 elapsed to enable an opinion to be formed as to the probability of 

 the plant becoming a permanent denizen in the British Isles, and 

 with this view a circular was sent to most of the corresponding 



