oS) 
to be found in lower pools in the summer and gradually 
spread upwards during the winter to form a conspicuous 
component of pools at much higher levels. The behaviour 
of Dumontia incrassata or of Scytosiphon lomentarius 
will illustrate the point—the latter being a well-marked 
component of the littoral zone. In the summer months 
it achieves optimum development about the mid-tide zone. 
The most luxuriant specimens die away towards the 
end of September but small individuals often without 
characteristic constrictions make their appearance at or 
near high tide mark and may be found there in large 
numbers, often without admixture of other algae, long 
after their fellows have quite disappeared from the 
lower levels of the shore. For some time these winter 
plants remain sterile, but in January and February they 
_ shew renewed activity of growth and reproduction and 
act as a source of “infection ”’ of all neighbouring pools. 
The successive generations now work downwards from 
the upper part of the shore towards low water level; at 
the same time the plants become successively larger and 
more luxuriant; they also reproduce readily. As the 
summer advances the upper pools become uninhabitable 
and the Scytosiphon plants die out from these levels 
during the months of July and August but re-appear in 
the autumn. There is thus, so to speak, a seasonal tide 
of Scytosiphon plants moving rhythmically from higher 
to lower levels of the shore and back again in the span of 
one year. This migratory habit ot Scytosiphon lomentarius 
is shared by other genera present in the flora throughout 
the whole year but appearing in different parts of the 
shore in different seasons. A comparative study of the 
flora of the limestone terraces of Port St. Mary at different 
times of the year has shown that many Rhodophyceae, 
especially species of Polysiphoma, Ceramium, Calli- 
thamnion move upwards during the winter. In January 
aC 
