23 
year. In February and early March the pools about the 
half-tide zone may be still in the denuded winter condition © 
but tufts of vigorously growing algae crown the heads 
of the limpet shells. These now serve as centres of 
distribution bringing about the rapid recolonisation 
of the pools in the early summer. 
ANNUALS 
Under this heading are included genera with a restricted 
vegetative period or those that appear sporadically ; 
the distribution of which varies markedly from summer to 
summer. These plants disappear completely at the end 
of the favourable season and cannot be accounted for 
by the persistence of fragments or of plant bases. Their 
re-appearance must be brought about by the liberation 
and subsequent germination of propagative cells of one 
sort or another. These plants are more strictly annual 
than any discussed above. 
In the absence of thick-walled resting spores (not 
produced by marine algae except perhaps by marine 
Vaucherias) it is difficult to conceive of a delicate 
unprotected unicell suffering the buffeting of the tides for 
five or six months and remaining capable of settling down, 
attaching itself to the rock and reappearing 1n recognisable 
form in the same locality where its predecessors had 
flourished. It must bea sine qua non that the reproductive 
cell attaches itself to the substratum very shortly after 
its release. The question to be answered is, therefore, 
to what extent does it develop in the autumn and in what 
form does it exist over the unfavourable period? An 
undeveloped spore or even a plantlet of two or three cells 
would have little chance of survival against the depreda- 
tions of browsing crustacea whose habit is to clean up 
any surface over which they progress. It seems more 
probable that the spores released in the autumn develop 
