FRESH-WATER ALG. 217 
upon lichens and old trees, giving them a rusty- 
yellow look. A remarkable species is found 
only on yew-trees of great age, growing in the 
deep clefts near the root, from which the juice of 
the tree exudes. Fed by that sap it increases from 
a black filamentous crust, to a thick corky sub- 
stance, which exhibits when broken a series of con- 
centric layers which indicate its age. When well 
dried it takes fire very readily from a spark, and 
burns like tinder. This singular substance, which 
is unlike any other plant, bears a greater resem- 
blance to a fungus than to an alga. I have 
gathered it on old yew-trees at Cleish Castle in 
Kinross-shire, and beside a ruined castle of the 
Macfarlanes in an island near the upper end of 
Loch Lomond. Upon the base of the abbot Mac- 
kinnon’s tomb in the ruined abbey of Iona, and 
abundant in Fingal’s Cave at Staffa, is found 
the purple Zrentepohlia, which looks like a piece 
of crimson plush or velvet, and exhales when 
moistened a sweet scent somewhat like that of 
violets. But the loveliest species of this family is 
T. pulchella, which grows on naked rocks or on 
aquatic mosses in mountain streams. It is of a 
rich violet or carmine colour, and imparts its own 
hue to whatever it comes in contact with. The 
stones and mosses in the bed of the stream where it 
occurs, look as if a dyer’s vat had been emptied 
