14
THE ESSEX NATURALIST.
tions on an extreme terrestrial form of this species, inhabiting
certain bare areas on Hindhead Common, in which the alga, no
doubt owing to an adaptation to its inhospitable substratum, may
be said to be permanently in the akinete-condition, its ceils
agreeing in many respects with the resting cells of species of
Zygnema.
A type of ground coloration which may perhaps be dis-
tinguished from the foregoing examples is that in which the
appearance is due to evident gelatinous masses of various hues.
These are all produced by alga, and the following three cases
may be taken as typical. One species of Nostoc (N. commune)
forms pale green masses of jelly of considerable size. West says
that it "often occurs in quantity on the surface of cultivated land
after a period of damp weather," and Mr. Harris tells me that he
has seen it forming extensive patches on wet roads. Small
brownish-purple gelatinous patches, due to Gloeocapsa magma, are
said to occur in great numbers on the ground and among wet
stones in parts of Western Scotland and the Hebrides, and the
gelatinous masses of Stigonema ocellatum, so Mr. Harris informs
me, are quite a wonderful sight in summer, when they stand up
in great black bosses above the shrunken surface of boggy places
on Woodbury Common, Devonshire. All three species men-
tioned are members of the blue-green algae, and in this connection
it may be interesting to note that in the tropics plants belonging
to this group are much more extensively developed under
subaerial conditions than in temperate regions. In a paper
published in the Geographical Journal, Prof. Fritsch has made
this point very clear, and he also shows what an important role
these subaerial algae play in the colonisation of new ground and
ven in the determination of scenery in the tropics.
There is just one other sub-aerial phenomenon caused by
microscopic organisms about which a few words may be said,
especially as I have already mentioned it at the beginning of this
address, and that is the occasional red coloration of snow. It is
very unlikely that this appearance is ever seen in this country,
but it occurs more or less regularly in comparatively small
patches on the higher snows of the Alps, Carpathians, Apennines,
Pyrenees and Scandinavian mountains, and is a much more
imposing feature of the Arctic and Antarctic regions. The red
colour is found to be due mainly to a small unicellular alga to