'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 sufificiently 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 



Fio. 109. — HydrochaHs Morsus-ranm (Froqbit). (Ajteb 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 the 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 Isoetes lacustris (quillwort, 

 Fig. 1 10), Lobelia Dortmanna (water-lobeMa), 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 ; Sparganium natans 

 (floating bur-reed), with floating leaves ; Potamogeton 



