8 



BRITISH CHAROPHYTA. 



many depths. As a pure community it is characteristic 

 in Derwent Water from 13 to 20 ft. In less dense 

 communities it occurs to depths of 24 ft. (Winder- 

 mere figures are slightly less than these.) Very 

 occasionally it includes Potamogeton pusillus (rare) 

 and P. prselongus (in small local patches) in depths 

 over 20 ft., but from 13-17 ft. often ousting the 

 Nitella. 



" In our experience Nitella does not fruit in these 

 deeper communities but does so freely in more shallow 

 water. This is probably due to the fact that the 

 light conditions are unfavourable the light-intensity 

 being only "035 to '025 of full daylight. The tem- 

 perature and C0 2 content are also known to be low 

 at these depths. 



" It should be noted that it is not the depth alone 

 which adversely affects these plants but the light- 

 intensity ascertained by us as the result of actual 

 experiments in these waters. The following figures 

 show that in our English Lakes the light- intensity is 

 constant at those depths where Nitella is dominant 

 and in dense masses. The waters, of course, are very 

 different; the bottom of Windermere is easily visible 

 at 20 ft., but that of Esthwaite invisible at 8 ft. 

 from the surface. 



" We are of opinion that the dominant factors 

 which determine the position of the communities are 

 the character of the inorganic silt and the rate of 

 sedimentation in the lake, not, as it is usually assumed, 

 primarily the mineral content of the water." 



