The Develoi^ment of Vegetation in the English Lakes, 271 



are much more abundant. Owing to the great development of Littorella and 

 Nitella, Isoetes is relatively scarce, while Jimcus Jiuitans, in its deep-water 

 form, is quite absent. P. perfoliatus and Elodea are becoming abundant in 

 the region formerly occupied by Isoetes {e.g., Derwentwater), while P. prce- 

 longus and P. pusillns are becoming abundant in the Nitella region. All of 

 these changes are attributable to the increase in abundance and fineness of 

 the silts. 



In Ullswater the abundance of Potamogeton causes a very marked reduction 

 in the quantity of Nitella. Here, again, P. perfoliatus is developing where the 

 Isoetes zone is becoming silted {i.e., in bays). If the development of these 

 lakes followed an exact sequence, Isoetes should be nearly absent from 

 Ullswater, as it is in Esthwaite ; but an impenetrable glacial till is common 

 in Ullswater, which, unless removed by wave action, prevents the extension 

 of Littorella, Potamogeton, or Nitella.. The persistence of Isoetes on these 

 primitive substrata is one of the features of Ullswater (and Coniston) and 

 partly accounts for the abundance of that species. The point will be 

 discussed in more detail below. 



Esthwaite, for which no diagram is given, shows a further advance on the 

 "Windermere stage, Isoetes being nearly absent, and Nitella encroaching on the 

 Littorella zone to depths of 1 to 1"5 m. In Esthwaite other higher plants 

 (particularly Naias flexilis) are numerous, to some extent replacing 

 Potamogeton. 



In primitive lakes higher plants only occur on abundant coarse fluvial 

 sediments in rather shallow water, and they are confined to a few localities in 

 each lake. Their development as deep-water plants along the shore-line only 

 occurs when silts become abundant, and there is a marked tendency for the 

 plants to progress into shallower water, already noticed for Isoetes and Nitella, 

 and clearly due to the increased silting of the shores. The case of P.perfoliatits 

 is a good instance of this point (see fig. 2). It is worthy of notice that the 

 first Potaviogeton to appear in quantity is P. perfoliatus, which normally occurs 

 on coarser and poorer silts than P. proilongus or P. pusillus. 



The examples considered above make it apparent that the zonation of 

 aquatic plants along a shore is an attribute of evolved lakes, and is, more- 

 over, primarily dependent upon the condition of the substratum as deter- 

 mined by the silting factor. Were one alone of these lakes to be considered, 

 it might well be assumed that zonation was due to light conditions, but such 

 a conclusion is difficult to reconcile with the series described above. This is 

 equally well shown when a series of lengths of shore in a single evolved lake 

 is examined. Generally speaking, the variations described above would be 

 found, provided conditions of exposure varied sufficiently. Thus, an exposed 



