50 THE ROYAL SOCIETY OF CANADA 



Characteristic dominant plants 



1 . Salix longifolia 



2. Scirpus fluviatilis 



3. Butomus tcmbellatus and Sagittaria hetero- 



phylla 



4. Vallisneria spiralis 



Elevation above water level 

 (18 Sept., 1921) 



(Correction: +13. 2' = reading on Lachine canal scale.)- 



Whereas the first two zones overlap, the others do not meet. 

 The intervening space between 2 and 3 is occupied by various annuals, 

 such as Cyperus esculentus, Bidens frondosa and Polygonum spp., not 

 having any ecological relation to water level. That between 3 and 

 4 is an open formation with scattered Butomus umhellatus,^ Sagittaria, 

 and Alisma. The Salix longifolia zone is quite typical of the St. 

 Lawrence beaches. It is about the only shrubby form that success- 

 fully meets the rigorous ecological conditions of seasonal submersion 

 by water and mauling by ice. Three characteristic plants accompany 

 the Salix in the lower half of its range Polygomiim emersum (hairy in 

 the upper levels, smooth in the lower), Spartina Aîichauxiana, and 

 Lythrum salicaria. In some other localities Spartina forms a distinct 

 zone between that of Salix and Scirpus ; here it occurs only as isolated 

 clumps. 



Other species abundant in the Salix zone are cocklebur {Xanthium 

 oviforme) and silverweed (Potentilla anserina). These have an 

 ecological relation to the water levels which need not be here dis- 

 cussed. But the presence of other common forms, such as Vicia 

 cracca and Desmodium with Cuscuta is probably fortuitous. 



In Zone 2. Scirpus fluviatilis is accompanied by S. americanus, 

 which has a slightly lower range. 



The dominant species of Zone 3 have also a slightly different 

 optimum, Butomus predominating above, Sagittaria below. The 

 rapid spread of Butomus so recently introduced is noteworthy. 



The fourth zone is almost pure Vallisneria. 



^Lachine canal scale reading for 17 Sept., 13.1'; for 19 Sept., 13.3', Montreal 

 Harbour Commission. 



^Thoroughly established in this locality. 



