26 
following a period of vegetative growth, greater or less 
according to the period of the year. 
As the vegetative period is extended the total mass 
of thalli of plants of one species naturally increases, so 
that when reproduction does occur it results in an 
increasing number of propagative cells at every successive 
crop. The proportion of the latter achieving successful 
germination also rises with the advancing season, thus 
accounting for the marked influx and sometimes over- 
whelming mass development of individuals of a given 
species in a particular area. The high point of this 
crescendo movement may be sharply defined. The 
beaches may be strewn in autumn with hundreds of 
plants of certain annual species, each in full reproduction, 
especially if a protracted period of rough weather super- 
venes in the early autumn. Within the space of a fort- 
night individuals of the species in question may almost 
entirely disappear. The change in physical conditions 
culminating in the autumn thus not only suddenly ~ 
exercises a deleterious effect on mature thalli but also 
markedly retards the germination of sporelings which thus 
remain dormant until the following spring. Such is the 
behaviour of most annuals. 
On the other hand, for some species, autumn conditions 
may be gentle in their operation so that a gradual 
diminuendo period follows that of maximum development. 
As the autumn weeks succeed one another, individuals 
of a given species occur less and less frequently ; they are 
smaller and smaller in stature and may not readily achieve 
reproduction. Even during the winter, however, indivi- 
duals here and there may still persist in sheltered places 
and small fragments may readily re-attach themselves. 
These characteristics most usually distinguish the pseudo- 
perennial. Thus the characteristics distinguishing the 
perennial, the pseudo-perennial and the annual algae are 
