T5 
PERENNIALS 
Under this heading may be included a number of 
genera and species whose representatives always form a 
conspicuous plant-covering to considerable areas of the 
littoral zone or of the shore below low water mark. This 
vegetation may remain 77 situ in a given locality year after 
year. For example, a boulder beach formed of stones 
too large to be moved easily by the tide will usually 
bear a covering of one or another species of Fucus ; while 
large boulders between tide-marks are constantly draped 
by Ascophyllum nodosum. The Fucus or Ascophyllum 
association is itself perennial but such a description 
can be applied only with reservations to the individual 
plants forming the association. 
Longevity is not a well-marked feature of algal life. 
The vigour of the individual soon wanes, and it is doubtful 
whether on British coasts the span of hfe of even these 
so-called perennials exceeds two or three years. The 
permanence of the vegetation in contrast to the imper- 
manence of the individuals is attributable to one or 
more of several factors :— 
(a) The daily liberation of enormous numbers of 
reproductive cells extending over a long period 
(often several months). 
(b) The ease with which the liberated cells attach 
themselves to the substratum and the readiness 
with which they germinate. 
(c) The power of proliferation from truncated basal 
discs or attachment organs of various sorts. 
The co-operation of these three factors is sufficient to 
maintain a constant supply of new plants to take the place 
of those that disappear. 
Perennation in the sea is thus somewhat different 
from what one understands by perennation on land. 
Marine algae have no embedded parts, well stocked with 
