8ia 



ZONES AND REGIONS [Pt. Ill, Sect. V 



instance, Lemnaceae appear to be absent from the lakes. Greater calmness 

 of water, special chemical constituents, and like factors may cause these 

 _^ differences, but no investigations have 



been conducted on the subject. 



The vegetation of fresh-water lakes 

 has been studied of late years with in- 

 creasing interest and success, but up to 

 the present only in Central Europe. 

 The following considerations, therefore, 

 chiefly relate to the lakes of Germany, 

 Switzerland, and the adjoining land. In 

 all lakes, benthos and plankton can be 

 readily distinguished, whereas shallow 

 bodies of water that possess only hemi- 

 plankton do not exhibit a clear differen- 

 tiation. 



i. The Limnetic Benthos of Lakes. 



Shallow banks of lakes are usually 

 separated from the mainland by a belt 

 of the common reed (Phragmites com- 

 munis), which towards the lake is con- 

 tinued by a belt of Scirpus lacustris. 

 Both plants are among those which have 

 only their bases in water and which we 

 have therefore distinguished as semi- 

 aquatic. They are frequently accom- 

 panied by other plants of a similar mode 

 of life, such as Butomus, Sagittaria, 

 Alisma Plantago, Ranunculus Lingua. 

 The shallow water between the tall shoots 

 is occupied by true aquatic plants of 

 smaller dimensions, which towards the 

 lake rapidly increase in number and size, 

 and when the depth reaches about 3 

 meters they entirely replace the others. 



The second belt, the first of the 

 true aquatic belts, is characterized in 

 the first place by Nymphaeaceae, and, 

 where all three Central European species occur, they succeed one another 

 in the following order: Nymphaea alba 1 , Nuphar luteum, N. pumilum 



1 I know nothing regarding the occurrence of Nymphaea Candida, Presl ; it is probably 

 not to be considered as specifically distinct. 



Fig. 498. Nuphar pumilum from Retour- 

 nemer Lake in the Vosges. Natural size. 



