9 4 PLANT GEOGRAPHY 



distribution of Algae what altitude is to that of mountain 

 floras, light playing a part similar to that played by 

 heat in the other case. An Upper Littoral Zone extends 

 down to low- water mark; a Lower Littoral Zone may 

 be traced down to a depth of 60-90 feet (10-15 fathoms). 

 Beyond this light is almost absent, and with it disappear 

 all the larger Algae. 



