208 FIRST FORMS OF VEGETATION. 
in moist, damp places, where they form a thin 
glossy-black pellicle of indefinite extent over the 
ground, strongly resembling, when dry, a piece of 
black satin (Fig. 19). Others are found in ditches 
and ponds ; a third species spreads extensively 
over damp walls in autumn and winter, a peculiar 
variety covering the damp walls in the inside of 
some Suffolk churches with bright sky-blue mould- 
like patches ; a fourth is often found on rotten 
timber, and trunks of aged trees where rain-water 
Fic. 19.—OsciLLaToriA NIGRA. 
trickles down. They may be found parasitic upon 
mosses in rapid streams, and forming thick glossy 
strata of a dull-brown or vivid-green colour, at the, 
bottom of clear, tranquil linns, wherever a film of 
soil is allowed to accumulate upon the naked slip- 
pery rocks. They are found in sulphur springs, 
forming pale yellow continuous tufts wherever the 
water retains sensible sulphureous qualities, as if 
the hepatic gas were necessary to their growth; 
and in the celebrated warm waters of Bath, a pecu- 
