218 FIRST FORMS OF VEGETATION. 
there. It is frequent in streams in Ireland ; but 
in England and Scotland it is somewhat rare and 
local. 
The extraordinary phenomenon of red snow has 
long been familiarly known to scientific men in 
this and other countries, and has naturally enough 
excited the greatestinterest. This singular colour 
in a substance with which we are accustoned to 
associate ideas of spotless purity and radiant 
whiteness, has been ascertained to result from an 
immense aggregation of minute plants belonging 
to the family now under consideration. They form 
the species called Protococcus nivalis (Fig. 20), in 
Fic. 20.—PROTOCOCCUS NIVALIS. 
allusion to the extreme primitiveness of its organi- 
zation, and the peculiar nature of its habitat. If 
we place a portion of the snow coloured with this 
plant upon a piece of white paper, and allow it to 
melt and evaporate, we find a residuum of granules 
just sufficient to give a faint crimson tinge to the 
paper. Placed under the microscope these gran- 
ules resolve themselves into spherical purple cells, 
