xxiv] AQUATIC ALIENS 303 



has vanished from various localities in Belgium, Holland and 

 Sweden, where there are records of its occurrence in compara- 

 tively recent times. It certainly seems to be a plant which is in 

 process of extinction in various parts of its range, since it 

 occurs in peat in a semi-fossil condition in places where 

 it has never been known alive within the memory of man 1 . 

 The exact reason for its disappearance is hard to find. 

 Probably the lowering of the mean temperature has some 

 bearing on the question, but it evidently does not provide 

 a complete explanation, since the Bull Nut can live in the 

 north of Scania, although that region is colder than Belgium 

 and the Swiss lowlands, where the plant is now almost, if not 

 entirely, extinct 2 . 



Just as certain terrestrial plants penetrate as weeds with 

 the seeds of cereals into alien localities, so aquatics find a con- 

 genial home in swampy rice fields, and are disseminated to other 

 countries in company with the rice. Thus the Lythraceous 

 Rotala indica and several species of Ammania^ belonging to the 

 same family, have penetrated into Kurdestan, Transcaucasia and 

 Astrakhan 3 , while Naias graminea has reached Upper Italy 4 in 

 the same fashion. The latter species has even been introduced 

 into England, probably with Egyptian cotton, and grew at one 

 time in a canal near Manchester, where the temperature hap- 

 pened to be artificially raised by the discharge of hot water 

 from various mills 5 . Cotton is probably also responsible for 

 the introduction into Yorkshire of Potamogeton pennsyhanicus, 

 which is the only non-native Pondweed recorded from Britain 6 . 

 In the Tropics, e.g. Fiji, a number of edible tubers, such as 

 Colocasia and Alocasia^ are cultivated at the borders of ponds 

 and ditches. It has been suggested 7 that aboriginal man, in 

 taking such moisture-loving food plants with him on his 



1 Reid, C. (1899). 2 Areschoug, F. W. C. (18732). 



3 Gin, A. (1909). 4 Ascherson, P. (1874). 



5 Bailey, C. (1884) and Weiss, F. E. and Murray, H. (1909). 



6 Fryer, A., Bennett, A., and Evans, A. H. (1898-1915). 



7 Guppy, H. B. (1906) and (1917). 



