The Development of Vegetation in the English Lakes. 269 



than upon variations in the composition of lake waters or in the physical 

 conditions of the habitat.* 



Table V summarises the soil types upon which the more important species 

 are found : — 



Table V. — Species and Soils. 



Plant 

 commumty. 



ISO. of 

 samples. 



Fine silt and clay 

 percentage. 



" Available " potash 

 percentage. 



Bate of 

 silting. 



Eange. 



Arerage. 



Eange. 



Average. 



Isoetes lacustris 



4 



5 -2-20 -8* 



11 -8 



-020-0 -023 



0-021 



Very slow. 



Juneus fluitans... 



5 



2 -2-14 -0 



7-5 



-023-0 -029 



0-025 



Moderate. 



Callitriche inter- 



3 



7 -4-16 -5 



12-6 







jj 



media 















Foiamogeton pei-- 



3 



10 -2-20-4 



15 -2 



-030-0 -034 



0-033 





foliatus 















P. pusillv.s and 



6 



12 -2-36 -8 



22 -5 



-034-0 -053 (4) 



0-042 



Eapid. 



P. prxlongus 













Naias jlexilis ' ... 





36 '5 and 46 "o 



41 -6 



-034 





J) 



* A little altered boulder clay. 



It will be seen that Isoetes and Juneus Jiuitans occur on the coarser soils, 

 poor in available potash, while Potaraogeton and Kaias are found on the finer 

 and richer silts. The decrease in abundance of Isoetes and Juncus in evolved 

 lakes can therefore be attributed to the development of Potamogeton spp., 

 consequent upon the increased abundance of finer and richer sediments. 



It has farther been shown that the development of silting on a lake shore 

 leads to the replacement of Isoetes by Nitella, which requires an easily 

 penetrable substratum. Where sediments are abundant and the mud deep, 

 higher plants (of group B) replace Nitella.* The gradual decrease of the 

 primitive plants from 85 to 30 per cent, in the lake series is due, therefore, 

 to the silting up of the shores in the evolved lakes, and to the greater rate of 

 sedimentation. 



It may be noticed, finally, that, including Elodea, the plants of the more 

 organic soils become more abundant and more varied in evolved lakes, which 

 have presumably been colonised by plants for the greatest period of time. 



The pecuharities of individual lakes may now be considered, to explain the 

 variations in Table IV. The frequencies given for any species are based on 

 the total records for the lake, reduced to the same relative proportions of 

 the lake flora as in Table IV. 



Tlie Development of Vegetation (fig. 2). — In Wastwater and Ennerdale, both 

 Isoetes and Nitella are most often found at a depth of about 6 m., and 



* Pearsall, loc. cit. 



