14 
the vanishing point in winter, are perhaps best described 
as “‘ pseudo-perennials.”’ 
Still another group of plants may be separated out from 
the rest, a group whose members are present, though not 
necessarily in abundance, for only a small proportion of 
the year, perhaps only during May, June and July; and 
which, despite careful search, are not to be found at all 
during the other months of the year. It is to such plants 
that the term “‘summer annual’”’ may most legitimately 
be applied ; though even here the term is used in a limited 
sense. This group includes such plants as Nemalion 
multifidum, Dumontia incrassata, Chordaria flagelliforms, 
Mesogloia vermiculata, etc. 
These three groups—the perennial, the pseudo-perennial 
and the summer annual—contain genera and species with 
definite stations or zones of distribution to which they 
remain constant year after year and in which they are to be 
found in greater or less profusion according to the season. 
A fourth group of plants may now be distinguished, 
whose occurrence is sporadic. These algae occur in small 
numbers or even as single individuals scattered here and 
there on the littoral zone. They are evanescent and 
there is no guarantee that having appeared in a given 
locality one year they will be found there the following 
year. Such plants are labelled “ rare ”’ in algal catalogues. 
Their presence in a list for a locality may rest on a single 
record and many years may elapse before another 
individual of the species is recorded for the same locality. 
These species may be described as “ casual annuals.”’ 
Associated with this group are also epiphytes or 
endophytes which are restricted to specific hosts, but 
are only of occasional occurrence. Sometimes the host — 
plants show no signs of the dependent plants; at others 
they are thickly tenanted by the invaders. Such parasites 
are classed as ‘‘ sporadic’”’ in the systematic list. 
