282 THE ESSEX NATURALIST.
as a rule, only a few dead algal cells were seen in the material
used for investigation, (2) that large numbers of algal cells
are always unattached to hyphae, (3) that the algal cells do not
divide vegetatively as is so generally assumed, and (4) that
sporulation frequently takes place within the thallus in the
same manner as in cells that have been isolated from the thallus
and subjected to cultural methods. These points are well
illustrated in the reproductions of photo-micrographs which
illustrate the present paper. (Plate XXIII. figs. 1 and 2,
Plate XXIV. fig. 3.)
Several stages in the process of sporulation of the gonidia
(algal cells) of Evernia prunastri are shown by photo-micrographs
which illustrate a paper by the present writer (26) on the sporu-
lation of the algal cells within the lichen thallus. Each fully
developed gonidium is potentially a mother-cell, and, as the
wall of each mother-cell becomes diffluent, during the liberation
of the daughter-gonidia, the absence of a large number of empty
cell-walls is accounted for. The presence of empty cell-walls
of gonidia within the lichen thallus has been advanced as an
argument in support of parasitism on the part of the fungus
symbiont.
An important paper by Dr. A. H. Church, entitled "Lichen
Symbiosis" (24) has been recently published. In this the author,
by the use of a quotation from the "Text Book of Botany,"
Strasburger, English edition, 1912, p. 417, directs attention to
the generally accepted meaning of the term symbiosis. He
maintains that the word, borrowed from zoological usage, is
a perfectly meaningless expression from the fact that any mean-
ing it is intended to convey has been lost in the vagueness with
which it has been applied. If "mutual advantage" is to be
understood it can be more clearly expressed as mutual dependence.
The statement, that in the case of the alga of a lichen, repro-
ductive organization is wholly wanting or omitted, does not
fully agree with recent evidence, gathered from published photo-
micrographs of the sporulating gonidia of Evernia prunastri,
referred to above (26). These clearly demonstrate that sporula-
tion of the gonidial cells (Chlorella) within the lichen thallus is
of common occurrence. It is more frequent within the thallus
than it is in free Chlorella, where sporulation has been described
as taking place only very rarely.