783 ZONES AND REGIONS [Pt. Ill, Sect. V 



perature of the water. The conditions of rainfall only come into considera- 

 tion, to a certain extent, in the case of fresh water. Of greater importance 

 in seas are the cold and warm currents, which also influence the terrestrial 

 flora, although only indirectly. 



As on land, so likewise in water, a vertical distribution is recognizable ; 

 to the regions of altitude of the land correspond the regions of depth in the 

 water. Yet we are concerned with entirely dissimilar phenomena. The 

 decisive factor in water is light, whilst to heat little or no importance 

 attaches. The regions of depth in water represent stages of decreasing 

 intensity of light — regions of illumination. It is advisable therefore to 

 express the fundamental importance of light even in the designation of the 

 regions. Three chief stages of brightness may be distinguished : — 



1 . The photic or bright region, in which the intensity of light is sufficient 

 for the normal development of macrophytes. 



2. The dyspJiotic or dim region, in which most macrophytes eke out 

 a scanty livelihood or cannot exist at all, while certain accommodating 

 assimilating microphytes (in particular Diatomaceae) still flourish. 



3. The aphotic or dark region, in which only non-assimilating organisms 

 can exist. 



In accordance with the unequal turbidity of the water, due to suspended 

 particles, the limits of these regions are at very different depths in individual 

 cases. 



In the air there is no floating flora, since bacteria and spores in atmo- 

 spheric dust are products of the terrestrial flora ; but in water a distinction 

 must be made between a fixed flora or bentlios, and a freely floating and 

 swimming flora or plankton. 



Both benthos and plankton show a differentiation into regions of illumina- 

 tion. Plankton is typically developed only above very deep water. In the 

 shallow water of coasts and of inland waters plankton is always mixed with 

 forms belonging to the benthos, and appears also in its own constituents to 

 be less independent of the soil. The floating and swimming plants of 

 shallow water will therefore be termed hemiplankton. 



Plankton and benthos in the sea are said to be pelagic, in fresh water 

 limnetic. 



The amount of salt, of heat, and of light determine the chief boundaries 

 between the regions in aquatic vegetation. To its detailed differentia- 

 tion other factors contribute, among which movement plays a prominent 

 part. The rapid succession of formations on the sea-coast is determined 

 not only by light and shade, but also by rest and movement (surf, ebb 

 and flow). Stagnant fresh water has a vegetation different from that of 

 running water. Water in motion demands in plants mechanical qualities 

 differing from those demanded by still water, and is also richer in air than 

 is still water. 



