106 TENTH ANNUAL REPORT OF 



In certain moist states of the atmosphere, accompanied bj a warm 

 temperature, the Nostoc grows very rapidly ; but what seems a sudden 

 production of the plant has possibly been long in preparation unob- 

 served. When the air is dry the growth is intermitted, and the plant 

 shrivels up to a thin skin; but on the return of moisture this skin ex- 

 panrls. bpforofs p-fla.tinons, and continues its active life. And as this 

 j^rocess is repeated from time to time, it may be that the large jelly 

 which is found after a few days' rain is of no very recent growth, A 

 friend of mine who happened to land in a warm dry day on the coast 

 of Australia, and immediately ascended a hill for the purpose of ob- 

 taining a view of the country, was overtaken by heavy rains ; and was 

 much surprised to find that the whole face of the hill quickly became 

 covered with a gelatinous Alga, of which no traces had been seen on 

 his ascent. In descending the hill in the afternoon, on his return to 

 the ship, he was obliged to slide down through the slimy coating of 

 jelly, v.'here it was impossible to proceed in any other way. No doubt, 

 in this case, a species of Nostoc which had been unnoticed when shriv- 

 elled up ha'1 merely expanded with the morning's rain. 



Where water lies long on the surface of the ground, as happens in 

 cases of floods, it quickly becomes filled with Confervce or Silk-weeds, 

 which rise to the surface in vast green strata. These simple plants 

 grow with great rapidity, using up the materials of the decaying vege- 

 tation which is rotting under the inundation, and thus they in great 

 measure counteract the ill efi'ects to the atmosphere of such decay. 

 When the water evaporates, their filaments, which consist of delicate 

 membranous cells, shrivel up and become dry, and the stratum of 

 threads, now no longer green, but bleached into a dull white, forms a 

 coarsely interwoven film of varying thickness, spread like great sheets 

 of paper over the decaying herbage. This natural pa^er, which has 

 also been described under the name oi lo at er flannel, sometimes covers 

 immense tracts, limited only by the extent of the flood in whose waters 

 it originated. 



But though AlgfB abound in all reservoirs of fresh water, the 

 waters of the sea are their peculiar home ; whence the common name 

 " Sea- weeds," by which the whole class is frequently designated. Very 

 few other plants vegetate in the sea, sea-water being fatal to the life of 

 most seeds; yet some notable exceptions to this law (in the case of the 

 cocoanut, mangrove, and a few other plants) serve a useful purpose 

 in the economy of nature. 



The sea in all explored latitudes has a vegetation of Algas. Towards 

 the poles, this is restricted to microscopic kinds, but almost as soon as 

 the coast rock ceases to be coated with ice, it begins to be clothed with 

 Fuci, and this without reference to the mineral constituents of the 

 rock, the Fucus requiring merely a resting place. Sea-weeds rarely 

 grow on sand, unless when it is very compact and firm. There are, 

 therefore, submerged sandy deserts, as barren as the most cheerless of 

 the African wastes. And when such barrens interpose, along a con- 

 siderable extent of coast, between one rocky shore and another, they 

 oppose a strong barrier to the dispersion of species, though certainly 

 not so strong as the aerial deserts ; because the waters which flow over 

 submarine sands will carry the spores of the Algte with less injury 



