24 
into minute cellular plantlets large enough to be resistant 
to forces tending to destroy them, possibly sheltered in 
tiny crevices of the rock, and ready with the return of 
Spring to develop into macroscopic plants. 
Just as in land plants there may be several crops of 
annuals in one growing season, if the life-cycles can be 
achieved in sufficiently short space of time, so, in the 
sea, the annual summer vegetation may really consist of 
large numbers of successive generations, rising to maturity 
and capable of reproduction in a very short space of time 
In the autumn, germination of released zoids takes place 
more and more slowly until finally the growth of the 
germling is so slow as to be quite imperceptible and the 
plantlet rests in a condition of protracted infancy until 
the following spring. 
Large numbers of filamentous forms and even genera of 
more ambitious organisation behave in this way. This 
method of existing through an unfavourable period is 
characteristic of summer Rhodophyceae, such as species 
of Nitophyllum, Bonnematsoma, Callithamnion, etc., and 
of many Cladophorae and other Chlorophyceae. The 
Phaeophyceae behave in a somewhat different manner 
and will be dealt with in a later section of the memoir. 
The constancy of these annuals in the flora is due to the 
fact that they produce propagative cells in large numbers ; 
the latter are sown broadcast over the littoral zone 
and on the substratum below low tide level and, being 
hardy, germinate in large numbers. Many of the plantlets 
are doomed to failure but those in an appropriate environ- 
ment. will readily replace their parents in the next summer 
population. 
CASUAL ANNUALS 
Casual annuals are only distinguished from “ annuals ” 
by the fact that they rarely occur. They are for the 
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