es 
seen to be differences of degree rather than of kind. 
Segregation of species into different categories is based 
on the following criteria :— 
(a) The extent of thallus destruction in the autumn. 
rt. Thallus only partially removed. Reproduction 
by sporelings and proliferation from basal 
parts of plants equally effective in producing 
new plants. Perenmaals. 
2. Thallus almost completely removed. Majority 
of new plants due to development of sporelings 
but proliferation from basal parts or from 
re-attached fragments also common. 
Pseudo-perennials. 
3. Thallus entirely destroyed. New plants due 
entirely to the development of sporelings. 
Annuals. 
(0) The extent of the growing period. 
1. All the year through. Perenmals. 
2. The major portion of the year. 
Pseudo-perennials. 
3. A restricted period in summer (or winter). 
Annuals. 
MEGAL MIGRATIONS IN THE LITTORAL ZONE 
From what has been said above it is clear that the 
algal population of the British coasts shows a certain 
periodic amplification and diminution corresponding to 
the alternation of summer and winter seasons. The 
direct effect of seasonal changes is felt less by perennials 
than by other classes of algae but rough water may be 
destructive of perennials, pseudo-perennials and annuals 
alike so that during the late summer numbers of plants 
may be detached and cast up. Autumn is frequently 
rendered conspicuous by the enormous masses of weed 
4 ‘ 
