VI] MARSH AND AQUATIC ASSOCIATIONS 159 



*Chara Braunii, *Naias graminea, and *Vallisneria spiralis. 

 These, as has been recently shown, are confined to certain parts 

 of the canal where the temperature is more or less permanently 

 raised by the inflow of heated water from adjoining cotton mills 

 (Weiss and Murray, 1909), and have no doubt been intro- 

 duced with imported cotton. * Potamogeton pennsylvanicus, 

 which, when first discovered in the canal at Halifax, was 

 thought to be restricted to the water which is heated by the 

 discharge from cotton mills (Bennett, 1908 a and b), has been 

 found to have a rather more extended range. This plant also 

 has in all probability been introduced with cotton from North 

 America. 



It is evident, therefore, that many water plants have ex- 

 cellent means at their disposal for successful migration. Not 

 only is this seen from the above-mentioned facts of distribution, 

 but an experiment (Wheldon and Wilson, 1907 : 339) confirms 

 this well-established opinion. A pond was made, near Garstang 

 in North Lancashire, in a grassy field to see which water plants 

 would appear. The pond was carefully railed off to prevent 

 access of cattle. After eighteen months, the following plants 

 had appeared : Nitella opaca, Callitriche sp., Alisma Plantago, 

 Glycerin fluitans, Juncus conglomeratus, and J. articulatus. 



It may, perhaps, appear surprising that practically no alien 

 aquatic plants should have become established on the Pennines 

 in the numerous reservoirs (see figure 36) which have been 

 constructed at the clough heads during recent years ; but the 

 reservoirs are artificially kept clear of water " weeds." However, 

 the only recorded station of the water purslane (Peplis Portula) 

 in the Peak District is on the south side of the reservoir at 

 Chapel-en-le-Frith (Linton, 1903: 151). 



The following is a list of the aquatic flowering plants of the 

 ponds, mill-dams, canals, etc. of the southern Pennines. Most 

 of the species occur only at altitudes below 600 feet (183 m.). 

 Those which occur at the higher altitudes are indicated by 

 the letter " U " (= upland) being placed in brackets after ' the 

 specific name ; and the plants that are not strictly indigenous 

 are preceded by an asterisk (*): 



