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most part species that have strayed from their geo- 
graphical area of distribution and, finding conditions 
generally somewhat unfavourable, are not successful in 
establishing themselves as constant components of the 
flora. The reproductive cells, though often produced, 
may find germination exceedingly difficult ; hence it is 
only an odd individual that succeeds in surmounting 
the rigours of the winter and reaching maturity in the 
following summer. Stilophora rhizodes will serve as an 
example. Naturally a member of a more southerly flora 
it is represented in the British flora of the South Coast 
and has been recorded for stations all round the coast of 
the British Isles, including the extreme North, but the 
Isle of Man record for the plant refers only to Port Erin, 
and as far aS we are aware no specimen has been found 
during the last eighteen years. 
One of the striking features brought out by a study 
of periodicity among marine algae is the readiness with 
which reproduction occurs. In the early spring it is 
quite common to find specimens of Polysiphonia, Cerannum 
or Callithammion bearing antheridia or tetraspores on 
thal that are barely half-an-inch long. Later in the 
spring, vegetative growth is carried still further before 
reproductive cells are formed, so that sterile plants 
two or three inches or even longer may be found. At the 
height of the growing season the limits of size achieved 
before reproduction takes place are still further extended. 
The co-existence, side by side, ot fertile and sterile plants, 
argues that the formation of reproductive cells is not 
necessarily a direct result of the physical factors of the 
environment but is associated in some obscure way with 
internal factors ; the net result of the combined internal 
and external forces being the precipitation of reproductive 
processes as the culmination of the plants’ activities, 
