332 



COMMON WEEDS 



small and monoecious, with many stamens, and one- 

 celled ovary. 



HYDROCHARIDACE^E 



Water-thyme : Canadian Pondweed (Elodea cana- 

 densis Michx. = Anacharis Alsinastrum 

 Bab.) is an introduced aquatic weed, 

 native of North America. The plant 

 (Fig. 95) is a fresh green colour, is 

 semi-transparent, and has completely 

 submerged, slender branched stems, 

 which bear whorls of three or four 

 lanceolate-serrated leaves at short inter- 

 vals. The stems are brittle, and often 

 3 or 4 feet long ; when broken in pieces 

 each piece is capable of developing roots 

 at the joints, so enabling it to attach 

 itself to the mud at the bottom of the 

 pond or stream, where it grows very 

 rapidly into a new branching plant. In 

 its native country Water-thyme is dioe- 

 cious, and produces small, one-seeded 

 capsules. In Britain only male plants 

 have been found, and these only in one 

 or two localities. It spreads vegeta- 

 tively, at an exceedingly rapid rate, by 

 means of broken pieces of stem when 

 introduced into ponds and slow-moving 

 streams. 



It appears to have been introduced 

 into the North of Ireland about 1836, 



'pondM and into En S lanc * ab out five years later. 

 dea canadensis From that time to the present it has 



Michx.), x$. 



made its way into all parts of the 

 country, and has become a constant source of trouble 



