158 VEGETATION OF THE PEAK DISTRICT [CH. 



ALIEN AQUATIC PLANTS 



A large proportion of the aquatic species occur in the canals 

 and mill-dams, and were not recorded by the older botanists 

 who flourished before these habitats were constructed. Hence 

 it is unlikely that such plants existed as members of the 

 primitive flora of a district which originally was almost if not 

 quite destitute of natural sheets of still waters. However, on 

 the construction of the canals and mill-dams about a century 

 ago, aquatic plants speedily invaded the new habitats, thus 

 proving that their previous absence from the district was only 

 due to the absence of natural ponds and lakes, and not to 

 any climatic reasons. 



Most of the aquatic plants in question were not introduced 

 here intentionally, but spread spontaneously; and these therefore 

 belong to a different category from such plants as the white 

 water-lily (Nymphaea alba) which has here and there been 

 intentionally introduced into the ornamental waters of some 

 of the parklands, as at Chatsworth. 



Many of the canals and mill-dams which harbour these 

 aquatic aliens are grossly contaminated with mill refuse ; but 

 a number of lowland aquatic species appear to be peculiarly 

 unsusceptible to such influences, perhaps because they have 

 become adapted to life in stagnant and often naturally foul 

 and badly aerated waters. On the other hand, those aquatic 

 plants which characterize swift-flowing streams, whose waters 

 are naturally well aerated and pure, are speedily extirpated 

 when the waters are rendered foul. 



The invasion of the artificial stagnant waters by aquatic 

 plants of the Pennines is still proceeding ; and quite recently, 

 an American pond- weed (* Potamogeton pennsylvanicus) has 

 become established in a canal near Halifax, a little to the 

 north of the present district. Mr A. Bennett (1908 a) states 

 that this is the first recorded instance of an alien Potamogeton 

 becoming established ; but all the Potamogetons of the canals 

 which cross the Pennines are, in a sense, aliens in these upland 

 waters. 



In a canal near Manchester, a few miles to the west of the 

 present district, such remarkable alien aquatic species occur as 



