POTAMOGETOX IN THE ENGLISH LAKES 163 



tion to the depth of water in which it grows, and since most of these 

 variations— if not all — have been reproduced experimentally by 

 growing the plant under varying light-intensities, it may be assumed 

 that light-intensity is the chief factor in causing them. In deep 

 water and low light-intensity the leaves are long, acute, and of a 

 clear translucent green (No. 282, K. & V.). In shallow water they 

 are much shorter, more obtuse, and of a dull olive colour (No. 238, 

 K. & V.). 



A comparison of two lots of material grown in water of the same 

 transparency and on similar soil gives : — 



1. Typical 2. Deep water 



,. . , . ,. P ond type. lake type. 



Depth in feet 3 2 •> 



Light intensity % 35 7 to 8 



Average leaf length * 307 mm. 45-4 mm. 



Average leaf breadth * 1-0 mm. 09 mm. 



Colour of leaves (K. & V.) 238 282 



Subsp. lacusteis (nobis). P. Sturrockii Ar. Benn. Kep B E C 

 841 (1919) ; nee Pots. Brit. Is. 85 (1915), nee Scott. Nat 28 

 (1883), nee P. pusillus L. subsp. Sturrockii Hooker, Stud Fl ed 3 

 435 (1884). 



_ Folia omnia submersa linearia, basi attenuata, apice rotundata 

 trinervia, ca. 30-50 mm. longa, 1-3-2-0 mm. lata. Liguhe hss* 

 convolute. Pedunculi breves ± 24 mm. Spica 2-3 mm. Fructus 

 non visus. Anatomia ut in P. pusillo. 



This beautiful and characteristic deep-water derivative of P 

 pusillus occurs in all of the larger English lakes, and is, in our 

 opinion, quite distinct from any other form of that species and from 

 P. Sturrockii Ar. Benn. (P. panormitanus x obtusifolius') It 

 shows no signs of either of the latter species in its ligules or ana- 

 tomy, and, moreover, P. panormitanus occurs only in Esthwaite 

 and P obtusifolius only in Windermere, Esthwaite, and Grasmere'. 

 It differs from all other forms of P. pusillus in its much 

 broader leaves of uniformly lighter colour, No. 277 (K &V ) 

 with apices very obtuse— usually nearly semicircular— and' in the 

 tact that it propagates itself solely by means of winter-buds It 

 is not P. pusdlus (type), for this assumes shorter and broader 

 leaves having thicker texture and darker pigments, in shallow 

 water, and in deep water very long and narrow leaves gradually 

 tapering into > an acute tip. Moreover, culture under weak liffht- 

 intensity (5 / or less) does not change the type pusillus into 

 lacustris , but only into the deep-water form as found in Crummock 

 1 inally lacustns shows but slight variations in relation to depth 

 and light-intensity, and it is not known from calcareous waters, 

 whereas pusillus affects both calcareous and siliceous types of water 



On the other hand— anatomically and superficially lacustris is a 

 pusillus derivative, and in accordance with this its first-formed 

 leaves are of the pusillus type in most essentials. We therefore 

 regard it as a subspecies -P. pusillus L. subsp. lacustris (nob.). 

 * Averages of fifty leaves. 



