8
THE ESSEX NATURALIST.
close contact, the daughter-cells gradually become rounded off,
and eventually detached, although before this happens they have
usually themselves commenced to divide—hence the common
appearance of groups of four cells looking somewhat like a
hot-cross bun with four indentations in the margin opposite the
arms of the cross. This vegetative reproduction appears to go
on indefinitely, and it is now considered very doubtful whether
any other kinds of reproduction occur or whether special resting
stages are ever produced.
Such then is a short description of the commonest form of
alga responsible for the pure green incrustation on tree trunks,
etc. But more often than not it is accompanied by a number of
other minute, more or less globose, unicellular algae, such as
species of Chlorococcum, Trebouxia, etc., and observations on
these have often been recorded as if they all belonged to the same
organism. In this way a great deal of confusion has arisen as
to the precise delimitation of the genus Pleurococcus and as to
whether there is more than one species. Further, the name
Pleurococcus has, in recent years, been abandoned by some authors
in favour of Protococcus, while one author, F. Brand, has intro-
duced two new generic names into the picture, viz., Desmococcus
and Apatococcus. Prof. Fritsch, who has gone into this matter
very carefully and who has most kindly allowed me to see proofs
of parts of his forthcoming book on British Fresh-water Algae is
of opinion, however, that the generic name Pleurococcus should
be retained for the common form of tree incrusting alga, but that
in future it must be known, not as P. vulgaris Meneghini but as
P. naegelii Chodat. Nevertheless from the painstaking observa-
tions of Brand it appears not impossible that further work will
show that even this species, narrowed down as it has been, may
comprise more than one type. However this may be it will be
sufficient for our present purpose if from now on we simply
allude to this alga as Pleurococcus, without troubling about the
specific name or names.
So far only the appearance, mode of reproduction and nomen-
clature of Pleurococcus have been dealt with. But there are
certain biological questions connected with this remarkably
successful alga which are worth a passing notice. For one thing
* This has now been published, the full title being "A Treatise on the British Fresh-water
Algae" by G. S. West, new edition by E. Fritsch (Cambridge University Press).