224 FIRST FORMS. OF VEGETATION. 
of rocks, and on the under surfaces of stones in 
various localities, and called by them Protococcus 
viridis, or green snow. It was also observed by 
Martins in similar situations in Spitzbergen. It 
is now, however, ascertained beyond doubt to be 
a mere variety of Protococcus nivalis, as it is 
identical with it in every respect save colour; 
and this difference is owing to the different cir- 
cumstances in which it is developed. The ac- 
tinic power of the solar light, aided by some 
peculiar, and as yet unknown property belonging 
to the natural whiteness of the snow itself, is 
highly essential in the production of the beautiful 
crimson or rose-colour, by which the red snow is 
distinguished ; but this colour, as in the case of 
the varieties mentioned above, gradually changes 
to green when secluded from the direct action of 
light, and developed on dark or opaque objects. 
Another extremely curious plant closely allied 
to the red snow is the Palmella cruenta (Fig. 21) 
or Gory Dew. Like the Protococcus it consists of 
a number of aggregated globose cells, forming a 
very thin crust-like frond of a dark blood colour. 
Each of the cells divides first into two, then into 
four parts, each capable of propagating the plant. 
It grows on damp limestone in the open air, or on 
whitewashed walls, particularly in cellars, and the 
mouldering rooms of old neglected buildings, and 
