Zygnemaceæ, and meet it by tlie assertion that, as far as our
experience goes, this suggestion is not supported, and the
evidence produced is far from satisfactory. The larg e r and
more complex hypothesis of the combination of Algæ and Fù n g i
in the production of Lichens, has been combated elsewhere,*
and to this we have nothing to add, save th a t we have since
encountered no facts or arguments which would induce us to
modify th at opinion. Even should hereafter the terrestrial
Nostocs be united with the Collemaceæ, the saxicolous Stigo-
nemeoe be relegated to the Sirosiphei, and the corticolous
Chroolepidce be transferred to some other relationship, the true
aquatic Algæ would still remain in sufficient number to interest
the student, and maintain for some time to come the distinctive
group of F re sh -W ate r Algæ.
* “ Grevillea,” Vol. vii., p. 102.
BRITISH
PHESH-WATEH ALGÆ.*
Algæ are usually associated under five classes, which are
taken in variable order, according to the judgment of the author,
and may thus be enumerated : —
I. Chlorophyllophyobæ, with the cell contents mostly of a
chlorophyll green.
I I . P hyooohkomophyobæ, with the cell contents mostly of a
bluish green.
I I I . Mbdanophyckæ, with the cell contents olive, brownish,
or blackish.
IV. R hodophyokæ, with the cell contents rosy, purple,
crimson or violet.
V. Diatomophyobæ, with an incombustible siliceous skeleton.
Other arrangements have been proposed, but the above will answer for
all practical purposes. The third class are all marine, and the majority
of the fourth, so that, exclusive of Diatoms, which are a special study,
the fresh water Algæ are mainly included in the first two classes.
C l a s s I . C H L O R O PH Y L L O PH Y C EÆ .
Plants aquatic or aërial, one, or two, or many-celled, eitber
single or associated in families. E ith e r branched or simple.
Cell wall not siliceous but combustible, sometimes composed of
successive layers, or strata, of a gelatinous substance. Cell
contents cbloropliyllose, sometimes crimson, flesh-coloured or
reddish-brown, often with a central or lateral nucleus. Starchy
granules very rare. Vegetation by cell division. Fecundation
often sexual. Propagation either by oospores or zygospores or
gonidia, which are motionless or agile {zoogonidia}.—Rahh.
Alg. ill. p. 1.
* Exclusive of Desmidieæ and Diatomaceæ.