2 
TIIE JOUllNAL OF BOTANY 
S. and E. of England. This difference is attributed to the less favour¬ 
able conditions for growth (see Pearsall, Journ. Ecology, viii. 1920, 
163). The species to be considered do not all vary to the same 
extent. Very variable species are P. perfoliatus , x P. nitens , P. gra- 
Qiiineus , and P. alpinus —and roughly in that order. P. prcelongus 
andP. crispus vary much less, while P. lucens and X P. Zizii show 
little variation, in our experience. The marked variability of P. 
polygonifolius as compared with the stability of P. natans is very 
striking and a familiar fact to most systematic botanists. 
P. pehfoliatus L. is the most widely distributed species in the 
English Lakes. The normal British form—with broadly oval leaves— 
found in calcareous waters in the south and east, does not occur 
in this area. Practically every lake contains a form of P. perfoliatus 
which differs in leaf-shape and size from those of the other lakes. 
Moreover, leaf-shape is by no means constant in any one lake. 
The following table summarises the differences between the typical 
forms of various lakes, compared with those of the more usual form 
from the K. Derwent at Kirkham Abbey, Yorks. The figures for 
length and breadth are averages of a number of leaves :— 
Lake. 
Length 
(cms.). 
Table I. 
Breadth 
(cms.). 
Length 
Breadth. 
Remarks. 
1. Esthwaite . 
... 2*4 
1-3 
1*85 
2. Derwentwater. 
2-2 
2*0 
3. Kirkham Abbey ... 
... 6-8 
3-4 
2*0 
Calcareous r. 
4. Windermere . 
... 3-3 
1*4 
2*4 
Water 3 m. dp. 
5. Ullswater. 
... 9*3 
3-6 
2*6 
f. macrophyllns 
6. Windermere . 
... 7-0 
1*7 
4*1 
Water 6 m. dp. 
7. Coniston . 
... 6*3 
1*4 
4*5 
f. lanceolatus *. 
8. „ . 
... 5*8 
1-5 
5*5 
In considering how these variations might have originated, it 
was immediately clear that light-intensity was one of the factors 
involved. We constantly found that the specimens with relatively 
longer leaves and internodes occurred in deeper water, and had hence 
developed under lower light-intensity. The effect of this is illus¬ 
trated by the Windermere specimens (4 and 6 in the table). 
While we could satisfactorily explain in this way many of the 
variations in leaf-shape observed in P. perfoliatus in any one lake, 
the effects of light-intensity alone were found quite inadequate to 
account for the differences in forms growing in different lakes. For 
example--the typical “ lanceolatus ” leaf-form in Coniston (No. 7) 
occurred in water about 4 m. deep. In Windermere the similar form 
(No. 6) came from water 6 m. deep, though these lakes have water 
of equal clearness (Pearsall, loc. cit.), and consequently the light- 
intensity will be similar at equal depths in each lake. Further, & the 
typical broad-leaved forms of Windermere, Esthwaite, and Derwent- 
water do not occur in Coniston, even in shallow water. The Kirk¬ 
ham Abbey specimens also illustrate the same points. The Derwent 
has turbid water, and the light-intensity is estimated to be about 
* Blytt’s so-called “ varieties.” 
