SECTION V. AQUATIC VEGETATION 



CHAPTER I 



GENERAL CONDITIONS OF EXISTENCE OF 

 AQUATIC PLANTS 



i. Halophytes and Non-halophytes. Salt, fresh, and brackish water. 2. Differen- 

 tiation of Aquatic Vegetation. Horizontal differentiation. Vertical differentiation. 

 Benthos, plankton, hemiplankton. Physics and chemistry of the substratum. 3. Periodic 

 Phenomena. 4. Special Consideration of the Factors. Salts. Temperature. Light. 



i. HALOPHYTES AND NON-HALOPHYTES. 



The influence of chemical factors on the distribution of vegetation is on 

 land subordinate to that of climatic factors, but in water it becomes of first- 

 rate importance. Every classification of the aquatic flora commences with 

 the separation oi salt-ivatcr forms horn fresh-water forms. These chief groups 

 are connected by the intermediate link of forms of brackish water, to which 

 those of estuaries and the majority of inland saline lakes belong. The dis- 

 tinction between the two extreme floras is obscured by that of brackish waters 

 only to a slight degree, as such waters are on the one hand of limited occurrence, 

 and on the other possess a scanty flora. 



Whereas scarcely a single terrestrial halophyte can be instanced that 

 cannot thrive in cultivation without or with only a trace of common salt, on 

 the contrary the transference of salt-water plants into fresh water, or the 

 reverse, is fatal in the vast majority of cases. The absence of terrestrial 

 halophytes in the non-halophytic terrestrial flora is due only to their inca- 

 pacity to struggle successfully ; the absence of aquatic halophytes in fresh 

 water depends on their unfitness to exist there. The difference between 

 halophytes and non-halophytes is therefore much more deeply seated in the 

 organization of water-plants than it is in land-plants. 



2. DIFFERENTIATION OF AQUATIC VEGETATION. 



Distribution in zones determined by the temperature of the climate is 

 much less prominent in water than on land, owing to the more uniform tern- 



