6 BRITISH CHAEOPHYTA. 



ties on the group. Though many species have a 

 wide-spread distribution only five have apparently 

 up to the present been found in all of the six con- 

 tinental areas. Of these areas probably Australasia 

 possesses by far the largest number of endemic 



species. 



As already stated, the Charophyta are 

 Conditions norma u y entirely submerged water- 



of growth. J Tr n -I , i 



plants. Very occasionally subterrestrial 



states of a few species are to be found growing in 

 bog-heads and in the nearly dried-up beds of small 

 streams. .They occur almost exclusively in still or 

 evenly-running water, being most frequently found 

 in ponds, gravel, clay, and peat pits, and ditches, but 

 also occurring in larger pieces of water, and occasionally 

 in streams and rivers. Water which is liable to much 

 disturbance, such as streams in mountainous country, or 

 which is subject to great changes of level, is unfavour- 

 able to their growth. They are rarely present in canals 

 or watercourses where there is continuous boat-traffic. 

 They will flourish only in clean water, not liable to 

 become turbid from the influx of mud, or to be con- 

 taminated by sewage or refuse from mills or factories. 

 Many species appear to flourish most in water 

 charged with a considerable quantity of lime, which 

 they take up readily in the form of incrustation, not 

 only on the exterior but beneath the cortex of th& 

 stem and branchlets. This calcareous incrustation 

 adds greatly to the rigidity of their structure. Some 

 species occur in brackish water, and a few actually 

 in the Baltic Sea, but that sea is particularly low in point 

 of salinity, being stated by A. F. W. Schimper to contain 

 less than one per cent, of salts. Certain species are 

 found only in water containing some marine salts,, 



