46 SCIENCE PROGRESS 



lobium, Xanthophytum, Musscenda, Lucincea, Urophyllum, 

 Lasianthus, Tarenna, Randia, Gardenia, Pavetta, and Morinda. 



Ecology. — W. H. Pearsall contributes an important paper 

 to the current number of the Journal of Ecology on the " Aqua- 

 tics and Marsh Vegetation of Esthwaite Water." A number 

 of plant societies are recognised related to differences in the 

 habitat conditions chiefly dependent upon the silt and organic 

 content of the substratum and differences in the intensity of 

 illumination. Exposure as a factor seems to be effective 

 mainly through its influence on the rate of sedimentation. 



The succession of the various societies depends on gradual 

 increase in organic content and the progressive rise in level of 

 the substratum. 



Litorella lacustris and Lobelia Dortmanna occur to a depth 

 of from *3-i'2 metres where the light intensity, in terms of 

 that above the water, rarely falls below o*i. The substratum 

 is usually gravelly, and at first unstable, but becomes fixed 

 by the Strapwort after which the Water Lobelia is able to 

 become established. In exposed situations, at a depth of 

 from 1*25-1*5 m., the Milfoil is dominant, associated with 

 Potamogeton alpinus and P. heterophyllus var. longipedunculata. 

 Isoetes lacustris is indicative of situations where the deposition 

 of silt is very slow, and occurs on rounded stones at a depth 

 of from 1*5-2*7 m. where there is a low intensity of light 

 (•06-03). 



A society of species characterised by their linear leaves 

 is found in similar light and depth conditions as Isoetes. It 

 reaches its full development, however, in sheltered situations 

 where there is mud containing under 15% of organic matter. 

 The chief species are Potamogeton pusillus (agg.), Naias flexilis, 

 and Callitriche autumnalis. Where the organic content is 

 very low (5*5-8*9%) Naias becomes dominant, whilst where 

 it is high ( 1 5*8-1 8*3 %) or silting is slow, Nitella flexilis flourishes. 



Under conditions of good aeration and high organic content 

 (18*3-19*3%) Fontinalis antipyretica is encountered. Spar- 

 gonium minimum is also found where there is considerable 

 organic material (21*3-23*6%), usually at depths from 2*4-3 m » 



The large white Water Lily grows to a depth of 2*5 m. in 

 sheltered situations where the organic content is high (23- 

 24*6%), but the small-flowered species (Castalia minor) seldom 

 occurs at a depth of more than 1 2 m. and is found where the 







