GELATINOUS, CRUST ACEOUS, AND STONE-LIKE ALG^E. 83 



their former consistency, even after they have been laid by 

 for a considerable time. 



Those species of Nostoc which are of a coarse dense 

 consistency, becoming corrugated when dry, may be 

 covered with stearine paper and placed under a light 

 pressure. 



Many of the Alga? again are furnished with crustaceous 

 or foliaceous fronds, firmly fixed to some underlying sub- 

 stance. Of this description are most of the Scy tonemacese, 

 the genera Hydrococcus, Schizosiphon, Chroolepus, Proto- 

 derma, Chlorotylium, Coleochaste, G-ongrosira, Hilden- 

 brandtia, Melobesia, and several others. Some of these (as 

 the Scytonemaceae and Chroolepus) form compact felt-like 

 layers on wet shady rocks and walls, or on the bark of 

 trees. Others (as Hildenbrandtia) are found encrusting 

 stones and pebbles, spreading irregularly over the surface, 

 and have a wrinkled leathery appearance. Finally, Hydro- 

 coccus, Chlorotylium, and some others, form tiny sponge - 

 like excrescences, which at length unite into a mass of rough 

 uneven knobs. Some of the smaller kinds (Coleochsety 

 Melobesia, &c.) are parasitic on the larger Algae, and such 

 water-plants as Sedges, Mosses, and Potamogeton, where 

 they often flourish in the greatest abundance, though 

 frequently overlooked, as many of them — the species of 

 Coleochaste, for instance— are too minute to be easily detected 

 by the naked eye. 



As it is always desirable to secure specimens that give 

 a clear conception of their mode of growth, the plants 

 should not be rubbed or scraped off, but gathered with 

 the object on which they are growing. Where the species 

 are at all abundant, small stones are sure to be found 

 clothed with the membranous crust peculiar to these Alga? ; 

 these may be taken and put away just as they are. In the 

 case of such as occur upon trees (the Chroolepi, for 

 instance), they may be obtained without injury by cutting 

 off a portion of the bark. They are more difficult to get 

 at when growing on the face of a rock or large block of 



Cx2 



