iv] THE CANADIAN WATERWEED 55 



verticillata, another member of the Hydrocharitaceae, has re- 

 cently been recorded from one station in Britain, though it is 

 typically a plant of warm climates 1 . But a fourth 

 genus, Elodea, represented by the Canadian 

 Waterweed, a submerged plant, which was 

 apparently introduced into this country about 

 i843 2 , has become very much more common 

 than any other member of the family. In nearly 

 all the localities in Britain, only the female plant 

 is found, though the male has been recorded 

 as occurring near Edinburgh 3 . The reproduc- 

 tion of Elodea canadensis, which is amazingly 

 rapid, is thus entirely vegetative; the snapping 

 of the slender, brittle stems sets free fragments 

 which live independently, while special winter- FIG. 34. Elodea 

 shoots may also be produced (Fig. 34). The %* 

 small leaves, which are arranged in whorls [Raunkiaer, c. 



r i 11 1 i i (1896).] 



or three, are only two cells thick and it is to 



their extreme delicacy that the plant probably owes its incapacity 



to produce a land form 4 . 



The pollination mechanism of the genus Elodea is of some 

 significance, owing to the different phases met with in different 

 species. Most of the species have inconspicuous flowers. The 

 male flowers either become detached and rise separately to the 

 surface of the water, e.g. E. canadensis 5 , or they are carried up 

 by the growth of their thread-like stalks, e.g. E. ioensis* 

 (Fig. 35, p. 56). The stigmas reach the surface owing to the 

 elongation of the floral tube which in E. canadensis may reach a 

 length of 30 cms. 7 . In an Argentine species, E. callitrichoides*, in 



1 Bennett, A. (1914). 



2 Marshall, W.(i852)and(i 857), Caspary,R. (18582) and SiddallJ. D. 

 (1885). See pp. 2 1 0-2 1 3 for a further account of the spread of this plant 

 in the British Isles. 3 Douglas, D. (1880). 



4 Schenck, H. (1885). 5 Wylie, R. B. (1904). 



6 Wylie, R. B. (1912), also E. canadensis according to Douglas, D. 

 (1880). 7 Wylie, R. B. (1904). 8 Hauman-Merck, L. (i9i3 2 ). 



