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 r 
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, T912, 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. 
