474 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



such waters and their plankton may usefully be described as 

 neritic. 



Halophytes are plants which can tolerate a considerable degree of 

 salinity. But whereas the land halophytes and those of brackish 

 waters are usually euryhaline (that is, able to tolerate a wide range of 

 salt-content in diiferent soils), most aquatic halophytes are more 

 stenohaline (capable of tolerating only a narrow range of salt-content 

 in different circumstances, their minimum, optimum, and maximum 

 being relatively close together). Similarly, eury photic and eurythermic 

 plants are those tolerating a wide range of light and temperature, 

 respectively, and stenophotic and stenothermic plants are those which 

 tolerate only a narrow amplitude in such respects. 



In general the chief boundaries between different types of aquatic 

 vegetation are determined by factors comparable with those operating 

 on land, though the emphasis is often changed. Thus temperature, 

 salinity, and light are of obvious importance in various ways, as are 

 movements due to surf and currents. Light-penetration depends 

 greatly on various factors such as cloudiness of the water (due to 

 suspended bodies whether living or dead), reflection from the surface, 

 latitude and the consequent angle of incidence of the sun's rays, 

 and content of dissolved substances. Moving water, besides being 

 better aerated, demands of plants mechanical qualities differing 

 from those required in still water, and, moreover, stagnant fresh 

 water tends to have vegetation of very different composition from 

 running water. Quite apart from this, shelter from currents and 

 waves can be an important factor and even a major necessity in 

 aquatic media. Currents, on the other hand, can disperse plants 

 and improve such conditions as aeration and the potentialities for 

 nutrition. Rainfall tends to be of significance chiefly in affecting 

 the salinity of lagoons, particularly in wet tropical regions. Aquatic 

 plants inhabiting such waters and the mouths of many streams must 

 be widely euryhaline ; thus certain Diatoms living in waters where 

 the salt-content changes widely and rapidly are able both to absorb 

 and let out salt equally quickly, according to the concentration of 

 the medium. 



We have seen that light-penetration in water is a very variable 

 factor. This is important because different Algae and other aquatic 

 plants needing light for photosynthesis vary greatly in their actual 

 light-requirements. As explained in the next chapter, this variation 

 is in part associated with the predominating colours of the different 

 groups, but it also exists in fresh waters where Red Algae are usually 



