Ll&KAK! 
NEW YORK 
SOTANiCAL, 
THE GARDEN 
J O U I 1 N AL 0 F BOT ANY 
BRITISH AND FOREIGN. 
POTAMOGETON IN THE ENGLISH LAKES. 
By W. H. Pearsall and W. H. Pearsall, I). Sc., F.L.S. 
Our former paper (Journ. Bot. 1921, 160-164) dealt with the 
2)usillus section of the genus Potamogeton, and it was there shown 
that “P. pusillus exhibits distinct variations in relation to the 
depth of water in which it grows,” and that “ light-intensity is 
the chief factor in causing them.” 
Similar variations occur in the larger-leaved species of Potamo¬ 
geton —and, indeed, in most other aquatic genera—and from similar 
causes. These will be indicated under the various species dealt with 
below. 
We also “ proposed to enumerate the species of Potamogeton 
found in the English Lake District and to summarise their distribu¬ 
tion.” This is, therefore, an additional object of the present paper. 
Since form variations in the larger pondweeds appear to be of the 
same types in all species, it will save repetition if these types of 
variation are summarised at the outset. 
The most important is the variation in the length and breadth of the 
leaf —most easily expressed as the ratio. This is associated 
with a change in the lengths of internode and peduncle—these 
becoming longer as the relative leaf-length increases. The factors 
producing these changes have been examined in detail in the case of 
P. perfoliatus , which shows this type of variation to a marked 
degree. The same factors—more or less—seem to operate also in 
P. prcelongus, P. alpinus , X P. Zizii, P. gramineus , X P. nitens , 
and P. crisp)us. Relatively longer leaves are produced in deeper 
water (i. e. from lower light-intensity and shorter duration), and 
broader leaves are found on the more calcareous soils. The chemical 
character of the salts dissolved in the water appears to have little or 
no effect. Light-intensity also affects the colour and structure of the 
leaves. In shallower water they become tinged with red and are 
several cells in thickness, instead of only two or three. The colour is 
due to the formation of anthocyanin consequent upon increased 
photosynthesis and sugar production. 
The average leaf area varies very considerably. Particularly in 
the case of P. perfoliatus , P. alpinus, and P. gramineus, the lake 
specimens have usually much smaller leaves than those from the 
Journal of Botany.—Vol. 61. [January, 1923.] b 
