238 BRITISH PLANTS 
of submerged plants by forming a dense covering through 
which no light can penetrate. In such cases there is 
usually a well-marked periodicity in the development of 
the vegetation. In the spring and early summer the - 
submerged plants grow luxuriantly, until the duckweed 
has multiplied sufficiently to form a thick covering, after 
which they cease to grow. In some places in the south 
of England a floating water-fern (Azolla caroliniana) has 
become established. This is a native of the southern 
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Fie. 109.—Hydrocharis Morsus-rane (Froasrr). (ArreR KERNER.) 
part of North America, and during the warmer months 
of the summer, when the temperature is about that of 
its native country, it grows so rapidly that even the 
duckweed is destroyed, and nothing is seen on the sur- 
face of the water but tne bronze-coloured fronds of the 
fern. 
The aquatic vegetation of highland lochs and pools is 
very poor, both in the number of species and individuals. 
Characteristic plants are Jsoetes lacustris (quillwort, 
Fig. 110), Lobelia Dortmanna (water-lobelia), and Subularia 
aquatica (awlwort)—all very similar in habit, with a short 
thick ‘ root-stock ”’ bearing a tuft of submerged erect 
cylindrical leaves, 2 to 4 inches long ; Sparganiwm natans 
(floating bur-reed), with floating leaves; Potamogeton 
