FIELD PHENOMENA DUE TO MICROSCOPIC ORGANISMS. 15
which many names have been given, but which is usually referred
to now as Chlamydomonas nivalis. The state in which it is found
on the snow is said to be the resting condition of the plant, but
this only refers to its lack of motility, for it seems clear that it
must be multiplying where it is found, otherwise it would be
impossible to account for the very well defined patches of colour
which it gives rise to and for the very different sizes of the cells.
The individual cells are almost perfectly spherical, and vary
in size from 1/2,400" to 1/800". They are of an intensely red
colour, owing to the presence of an oily material containing
haematochrome. There appears to be a small more or less central
mass of chlorophyll, but this is usually almost entirely masked by
the red oil. In the specimens I found in Savoy last year practi-
cally every cell was surrounded by a layer of irregular particles,
probably fragments of minerals which had been blown on to the
snow as fine dust from the surrounding rock surfaces, and which
had adhered presumably to a thin gelatinous outer covering of
the cell. It is tempting to surmise that it is in this way that
these plants obtain such mineral matter as they require, for it is
difficult to understand in what other way they could get such
material on a substratum like snow. The vivid red colouring
matter is undoubtedly an adaptation to the peculiar habitat of
this organism, either in connection with the low temperature or
the intense insolation to which it is exposed, or perhaps to both.
A few other organisms are usually found associated with the
typical red snow alga, and yellow, green, brown and black snows
have been recorded as due, in part at any rate, to algae, but
these are much less frequent than red snow.
We now come to the consideration of those field phenomena
dependent upon microscopic organisms which occur in the waters
of our lakes, ponds, streams, etc. There are several categories
of these, and I propose to refer first of all to the cases in which
the water itself appears to be coloured by reason of the organisms
being present in immense numbers and being distributed through
it in a nearly uniform manner.
Green water may be caused by a great many microscopic
organisms, all of them probably to be considered as plants,
although some are even to-day claimed as animals in many
zoological text-books. The most usual of the organisms pro-