[ 2IO ] 



CHAPTER XVII 



THE VEGETATIVE REPRODUCTION AND 

 WINTERING OF WATER PLANTS 



THE conditions under which hydrophytes live unlimited 

 water supply, abundant carbon-dioxide and protection 

 from sudden temperature changes are favourable to acti- 

 vity of growth^, and the luxuriance which this vegetation often 

 attains is a matter of common note; indeed it sometimes 

 becomes such a hindrance to navigation as to compel the atten- 

 tion, not only of botanists, but also of those who normally take 

 no interest in plants. Even in the rivers of countries with a 

 temperate climate, such as our own, aquatics are liable to 

 multiply at a rate which renders them a great embarrassment in 

 boating. A letter, for instance, which appeared in the Morning 

 Poj/of July 1 6, 1 9 14, refers to a locality in the Thames above 

 Whitchurch Weir, where the weeds were "20 ft to 30 ft long 

 and close under the surface of the stream from one bank to 

 another." The sluices of mills are liable to be choked, too, in 

 the autumn, by the countless detached fragments of Potamo- 

 geton. 



But the classic example in England of the extremely rapid 

 growth and multiplication of a water plant, is the behaviour of 

 Elodea canadensis'^^ the American Waterweed, in the first decade 

 or so after it made its debut in this country. Exactly when and 

 how it was introduced from America remains a mystery. Its 

 first appearance in Great Britain is said to have been in Ireland 

 in 1836, while its first recorded occurrence in England was in 

 Berwickshire in 1842^. It travelled south, and by 1851 was so 



iSchenck, H. (1885). 



2 Marshall, W. (1852) and (1857), Caspary, R. (18582), and Siddall, 

 J. D. (1885). For the continental history of the plant see Bolle, C. 

 (1865) and (1867). 3 Johnston, G. (1853). 



