FIELD PHENOMENA DUE TO MICROSCOPIC ORGANISMS. 7
the other of a greyish-green colour. The former is produced by
a nearly pure algal growth, while the latter is due to a mixture
of algal cells and fungal hyphae, in more or less close association,
to a lichen in fact, in a rudimentary state of development. And
here I should explain that although it is not my intention to
refer in this address to field phenomena due to lichens, mosses and
fungi, even when these are of such small dimensions as to justify
such a procedure, I cannot help making an exception in the case
of the powdery lichens which occur as incrustations on tree
trunks and similar situations. The components of these are in
such a primitive stage of association that the algal cells (the
so-called gonidia) are often practically free from enveloping fungal
hyphae and do in fact sometimes form almost pure incrustations
on their own account.
As regards the pure green incrustation it will be found, in
nine cases out of ten, that it is due to the minute alga usually
referred to in the text-books as Pleurococcus vulgaris or, less
frequently, as Protococcus viridis. Under the microscope the
powdery material is seen to consist of masses of green cells,
on the average about one three-thousandth of an inch in diameter,
which may be either solitary, when they are approximately
spherical, or more often joined together in groups of two, three,
four, or occasionally more, the group of four being perhaps the
most usual. The isolated globose cells must be regarded as the
mature form of the alga, while the groups are evidently made up
of cells which have not yet completed their separation after
division. All the cells and cell groups adhere more or less
tenaciously to one another, but the exact nature of the binding
medium is not known. It does not appear to be gelatinous,
but may be due to an extremely thin coating of some resinous,
waxy or gummy material. The walls of the cells are quite
smooth, and although well marked are not, as a rule, noticeably
thickened. The chloroplast more or less closely approaches
the cell-wall and lines nearly its whole inner surface. There is
no pyrenoid, but minute spherical granules, probably of an oily
nature, are often to be seen between the chloroplast and the
cell-wall. Cell-division takes places by a septum being formed
across the middle of the cell, the daughter-cells being thus
clothed in part by half of the wall of the mother-cell and in part
by the septum. At first necessarily nearly hemispherical and in