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 aérated waters. On the other hand, those aquatic 
plants which characterize swift-flowing streams, whose waters 
are naturally well aérated and pure, are speedily extirpated 
when the waters are rendered foul. 7 
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 
