10
THE ESSEX NATURALIST.
than itself, but occasionally forming almost pure green powdery
growths by themselves. The most usual, perhaps, are species
of the genus Stichococcus. The patches of incrustation formed
by these are usually of a yellower green than those of Pleuro-
coccus, and as a rule are found nearer the ground and in more
protected situations on the tree trunks. When examined under
the microscope the individual cells are seen to be more or less
elongated rods, on the average about 1/2500" in length, with
the chloroplast going only part of the way round the cell-wall, as
in many species of the filamentous alga Ulothrix. The cells,
although usually separate, are very frequently joined end to end
into short filaments of two or more, and there can be no doubt
that the species of this genus are descendants of aquatic Ulothrix
types, which have retrogressed towards the unicellular state,
possibly during the process of their adaptation to sub-aerial
conditions.
I pass now to the grey-green incrustation on trees, etc.
Although this is produced by a lichen, usually, as I understand,
Lecanora varia, I want briefly to refer to it, as the algal cells and
fungal hyphae are in such a rudimentary state of association
that we can almost regard them as still independent microscopic
organisms. The algal cells are rather larger than those of
Pleurococcus, being on the average about 1/2000" and they are all
nearly spherical. The chloroplast, usually more or less lobed,
is placed centrally and it contains a very distinct central body
which is probably a pyrenoid. Reproduction, unlike what
occurs in Pleurococcus, takes place by the formation within the
mother-cell of from four to sixteen aplanospores or autospores,
which are provided with their own cell-walls and are only liberated
by the rupture or absorption of the wall of the mother-cell.
There has been some divergence of opinion in the past as to the
genus to which this alga belonged, and it has been referred to
Protococcus, Chlorococcum, Chlorella, etc. Recently, however, it has
been placed in a special genus, Trebouxia, named by Puymaly
after Treboux, who was the first clearly to distinguish its special
nature. The same or an exceedingly closely allied form occurs
in a great many lichens and possibly all may be regarded as
T. humicola Treboux. In the grey-green powdery material on
tree trunks, etc., the cells of Trebouxia are nearly all more or less
surrounded, either singly or in small groups, by the hyphae of the