156 VEGETATION OF THE PEAK DISTRICT [CH. 



related to the absence of a large number of lowland aquatic 

 flowering plants. The shallowness of the streams assists 

 towards the same result. The streams on the sandstones and 

 shales are deficient in humous acids, except in the case of the 

 streams which drain the peaty deposits of the plateaux. The 

 streams of the limestones are not merely deficient in humous 

 acids, but are actually alkaline in reaction. 



On the flatter moorland plateaux in streams draining from 

 the peat moors, a pond weed (Potamogeton polygonifolius) is 

 indigenous. This plant appears to be confined to acidic waters. 

 A crowfoot (Ranunculus Lenormandi) is locally abundant in 

 similar situations, as well as in small streams and swamps on 

 the hill-slopes of the sandstones and shales. JR. hederaceus is 

 very much rarer. Glyceria fluitans and Callitriche stagnalis 

 are found in the streams, in reservoirs and mill dams, and in 

 swamps up to 1500 feet (457 m.). Montia fontana, M. minor, 

 and Stellaria uliginosa are also locally very abundant in the 

 streams on the sandstones and shales. The Batrachian crow- 

 foots are almost confined to the limestone streams, where, 

 however, they are locally very abundant, especially so below 

 500 feet (152 m.). Species of Chara are also limited to the 

 calcareous streams, whilst Nitella opaca occurs rarely in both 

 the calcareous and the non -calcareous waters of the district. 



Ostenfeld (loc. tit.) includes "the vegetation on cliffs over 

 which water constantly trickles " with the vegetation of round 

 springs and streamlets, and this appears quite a reasonable plan; 

 and Ostenfeld (loc. cit.) is also quite reasonable in refusing to 

 follow Jonsson in placing this type of vegetation with the moss- 

 bogs described by Warming, which are characterized by stagnant 

 and not by well aerated water. 



On the whole, however, flowering plants are not generally 

 abundant in quickly flowing streams; but this deficiency is 

 more than compensated by the great abundance and variety 

 of liverworts and mosses. Bog-mosses (Sphagnum spp.) never 

 occur in the streams of the limestone hill-slopes, and are not 

 common in those of the siliceous hill-slopes except when these 

 are rich in humous acids. Algae are locally abundant and 

 equally characteristic, but have not been studied ecologically. 



The following liverworts and mosses have been observed in 

 the streams of the southern Pennines : 



