THE BASIC ZONATION 



315 



flora of the Isle of Man. They showed, for example, that successive 

 generations of Cladophora rupestris move vertically up and down 

 5-8 ft. each year, the movement being rendered possible because 



there is a monthly reproduction when the sporelings only survive 

 in the most favourable zone for the particular time of the year, and 

 this is not necessarily that in which the parent plants are growing. 

 Some algae migrate up in winter and down in summer, whilst 

 others move up in summer and down in winter. It is suggested that 

 the nature of the response to temperature controls the movements 



