32 
and the species “disappears”’ from the flora of the 
upper pools, though it may still be flourishing in pools 
further down the shore. Later on adverse conditions may 
affect successively deeper lying pools, so bringing about 
what appears to be a “ migration.”’ In the following 
spring the procession of macroscopic plants follows the 
reverse order and appears to rise from level to level as 
spring merges into summer. 
Many species of Rhodophyceae (Rhodomela subfusca, 
Delesseria sp.) and some Chlorophyceae undergo this 
seaward migration in the winter. The pools about high 
water of neap tides are characteristically green in summer. 
Many of them are inhabited exclusively by species of 
Enteromorpha, Ulva, Cladophora, Rhizoclonium and 
Chaetomorpha. During the winter this well-marked zone 
is almost obliterated and representatives of the genera 
in question are to be found at much lower levels—in 
the mid-tide area for example. The behaviour of 
Cladophora rupesivis may be most marked ; in September 
it forms, on the lmestone terraces of Port St. Mary, 
a well-marked zone several yards in depth at a distance 
of 50 yards (representing a drop in level of 12 feet) from 
high water of neap tides. Observation of this zone in ~ 
January shows that the plants are worn down to their 
bases or completely removed and the well-marked zone of 
September was scarcely detectable, though large numbers 
of plants actively growing and already reproducing were 
to be found further down the shore nearer to low water 
mark. The zoids from the plants in this area are doubtless 
responsible, together with proliferation from basal remains, 
for the re-establishment of the mid-tide Cladophora 
vupestris zone in the early summer. 
The direction of these migrations is not always as 
outlined above, for there are many species whose move- 
ments are in exactly the opposite direction. They are 
