POTAM0UETON IN TILE ENGLISH LAKES 
3 
the same at a depth of 2 in. as that of Windermere at 6 m; The 
lowermost leaves of a large number of specimens were 9 cms. long 
and 2 cms. broad—the uppermost being 3 3 cms. long and 2*8 cms. 
broad. The former were growing in 2 m. of water, while the latter 
were only 20 cms. below the surface. Clearly the leaves are rela¬ 
tively much broader than any of the lake specimens growing under 
similar light-conditions, and never does P. perfoliatus in the lakes 
produce leaves in which the length is only 1*2 times the breadth— 
or indeed anything approaching this ratio. 
In 1920 we took material of the three extreme types from 
Coniston, Esthwaite, and Kirkham Abbey, and grew it in large 
tanks (5 ft. diameter and 5 ft. deep; =T5 m.) under the same 
light-intensity and on the same soil. All three sets of material pro¬ 
duced leaves of the same type (about 3 cms. long and To cm. broad), 
but on growing these further under different light-intensities, we 
were not able to produce the extreme types of leaf from which we had 
started—although in half the original light-intensity the leaves were 
much narrower (0'9 mm.) and somewhat longer (3'3 cms.). The 
approach of winter put a premature end to this experiment, but the 
results obtained seemed to indicate quite clearly that another environ¬ 
mental factor—at least—affected the leaf-shape of P. perfoliatus , 
and the nature of the soil seemed to be the most probable additional 
influence. The fact that the broad-leaved Kirkham Abbey form 
occurs on very calcareous soil (-j-5°/ 0 of CaC0 3 ) seemed very 
suggestive in this connection, and so far as we could ascertain, all the 
typical broad-leaved southern forms of perfoliatus grow on calcareous 
soils. We had a limited number of soil analyses—from the ecolo¬ 
gical survey of the lakes—and on referring to these we found, in 
accordance with the Yorkshire example, that the broader-leaved lake- 
forms grew on the most calcareous soils and the longer narrower 
forms on the least calcareous soils as shown by the following 
table (II) :— 
T „ length 
Lake. ^ breadth. 
Coniston . 4'5 to 5'5 
Ullswater . 2‘6 
Esthwaite . 1'8 
Available in 
soil # . 
Ratio. 
K.,0 
CaO 
K O 
(Potash). 
(Lime). 
CaO. 
338 
90 
3-7 
336 
280 
1*2 
342 
1600 
0-2 
These figures, although limited in number, are very suggestive 
when viewed in connection with the facts outlined above. They are 
confirmed by a similar series for P. prcelonr/us (see below), and 
suggest quite definitely that the proportion of lime in the soil is an 
important factor in determining leaf-shape in P. perfoliatus , and 
that, along with the effects of light-intensity, it may enable us to 
account for all the leaf-variations observed in this species in nature. 
Further experiments to analyse these effects are in progress—they 
confirm the above conclusion—but open up additional problems which 
prevent any present statement of results. It seems quite certain 
that there are no “varieties” in P. perfoliatus , but only growth- 
forms—the view held by Fryer. 
* Parts per million of dry soil. 
