POTAMOGETON IN T1IE ENGLISH LAKES 
5 
xP. angustieolius Presl (=P. Zizii Mert. et Koch) is a 
hybrid of P. gramineus xlucens. At present it is only found in 
Derwentwater and Coniston. In the former it grows in about 1 m. 
of water, and the leaves have a reddish colour owing to the presence 
of anthocyanin. These specimens also fruit freely, and both these 
distinctive features can probably be attributed to high light-intensity 
(±85%) and a consequent high rate of carbon assimilation. The 
Coniston specimens—growing in water about 3 m. in length (L. I. 
±10%)—lack both the colouring and the fruiting of the Derwent- 
water plant. 
Dr. Hagstrom (Crit. Bes. 212) remarks that “its fruiting-faculty 
appears in some cases nearly quite undiminished or normal, in most 
cases considerably reduced, and in many quite lacking.” While this 
accords with our own experience of this species, we may add that the 
fruiting-capacity—as in the case above—seems to depend very largely 
upon the light-intensity and the rate of carbohydrate synthesis. If 
this is high, abundant fruits appear, and in our judgment this is 
usual among the larger-leaved species of Potamogeton. 
The leaves of X P. angustifolius show little change in form in 
the habitats from which we have gathered them. The average size 
of the Derwentwater specimens is 14 cms. long x 2*8 cms. broad, the 
Coniston specimens being smaller, but having the same average pro¬ 
portions, e.g ., 10 cms. long x2 cms. broad. 
P. alpinus Balbis ( = P. rufescens Schrader). This species is 
easy to distinguish when the characteristic coriaceous floating leaves 
are present. These are obovate in shape, cuneate at the base, and 
have petioles shorter than the laminae. In the lakes, however, this 
species—in common with many other species of Potcimogeton —rarely 
produces floating leaves, but the reddish tinge of its submersed leaves 
readily distinguishes it, 
Two distinct forms are met with—(1) that of deep water and low 
liglit-intensity. This possesses much longer leaves (10-12 cms.) of 
thinner texture, and (2) the shallow-water form which has much 
shorter leaves of thicker texture—often, indeed, subcoriaceous. The 
first occurs in Derwentwater, Ullswater, Esthwaite W., Windermere, 
and Blelham Tarn. The second is found in the shallow water at the 
Blelham Beck in Pull Wyke Hay, Windermere. This beck flows 
from the tarn of the same name, and the difference in habit of the 
alpinus plants growing in each water is primarily conditioned by a 
difference of light-intensity consequent upon difference of depth in 
the respective habitats. The two widely different forms pass into one 
another so gradually in the varying depths of water in the tarn, the 
beck, and the lake, that it cannot be doubted that they are merely 
“ states ” produced by local—and indeed, often changing—conditions. 
[ X P. lanceolcitus Smith, given in Hodgson’s FI. Cumb. 315 as 
occurring in Bassenthwaite was obviouslv an incorrect determination. 
Neither this nor any similar species is found there.] 
P. gramineus L. ( = P. heterophyllus Sclireber) prefers rela¬ 
tively shallow water and some degree of shelter. Under such con¬ 
ditions the ordinary form of the species flourishes in Ullswater, 
Coniston, Windermere, and Lowes water. Occasionally, however, a 
