270 



Mr. W. H. Pearsall. 



have similar ranges of depth. The plants are normally rather scattered, 

 and obviously colonising bare areas. Very large parts of the lake shores 

 are devoid of vegetation. The depth distribution of Isoetcs and Nitella i& 

 remarkable when it is considered that, while the light limit for vegetation 

 in these two lakes must be at least 10 m., probably more, yet little- 

 vegetation occurs below 7 m., or, normally, above 3 m. The development 

 of vegetation between these two depths is undoubtedly due to the fact that 

 wave-formed sediments are deposited more commonly at 4 to 7 m., while 

 above this zone rocks are found, and below it rocks or glacial till. In well 

 sheltered bays or near streams, the zone of silts may be much more 

 extensive, and there the vegetation is correspondingly more widely spread. 

 On the abundant sands near stream mouths, Junaos fluitans replaces Isoetes 

 and Nitella, and Littorella may be found locally on wave-beaten gravel. 

 Normally, however, there is little zonation of different types of vegetation. 



Great differences are apparent when the Derwentwater figures are examined. 

 Nitella is still most abundant at the same depth, but its downward extension 

 is curtailed by light conditions. It is becoming more abundant in shallow 

 water and, as in Windermere and Ullswater, its upper limit coincides roughly 

 with that of the mean silted area. Eocks are rarely found below 5 m. Isoetes 

 is now most abundant in much shallower water — 2-3 to 3"6 m. — and has 

 obviously been replaced at greater depths by the combined effects of silting 

 and Nitella competition, while its further extension into shallow water is; 

 stopped by the development of the Littorella zone on marginal gravel and 

 sand. It is, in fact, confined to the least eroded and least silted parts of the 

 shores, i.e., the most primitive. This is the usual type of distribution of 

 Isoetes and Nitella in these lakes, and it is diie to the physical conditions of the 

 lake shores. Where, as in E-nnerdale, the moulding and silting of the shores 

 has progressed but little, and the waters are very clear, one may find Isoetes 

 below Nitella as well as above, the Nitella coinciding with the zone of 

 maximum silting. A similar less frequent example in Ennerdale gives : — 



(1) Littorella, abundant on gravel and sand to 4 m. 



(2) Nitella, abundant on deep fine sand and silt, 3 to 5 m. 



(3) Isoetes abundant on little altered glacial clay, 5 to 8 m. 



Eeturning now to Derwentwater, here again the higher plants, Jtmcus 

 Jiuitans and P. perfoliatus, are chiefly confined to shallow water on abundant 

 sediments near stream anouths, the appearance of P. perfoliatits heing attributed 

 to the finer nature of the silts. Small quantities of Potamogeton are also 

 appearing in deep water, replacing Nitella. 



In Windermere all these processes have progressed much further, as silts. 



