rV. INTRODUCTION. 19 



and the only reason for regarding it as an aerial visitant is the suddenness of its 

 appearance after rain. 



In certain moist states of the atmosphere, accompanied by a warm temperature, 

 the Nostoc grows very rapidly ; but what seems a sudden production of the 

 plant has possibly been long in preparation unobserved. 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 expands, becomes gelatinous, and continues its active life. 

 And as this process 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 obtaining 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 tlie 

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

 impossible to proceed in any other way. No doubt, in this case, a species of Nostoc 

 which had been unnoticed when shrivelled up had 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 Confervm or Siik-iveeds, which rise to the surface in 

 vast green strata. These simple plants grow with great rapidity, using up the 

 materials of the decaying vegetation which is rotting under the inundation, and 

 thus they in great measure counteract the ill effects to the atmosphere of such 

 decay. When the water evaporates, their filaments, which consist of delicate mem- 

 branous 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 

 pajyer, which has also been described under the name of ivafer flannel, sometimes 

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

 originated. 



But though Alg£B abound in all reservoirs of fresh water, the waters of the sea 

 are their peculiar home ; whence the common name " Seaweeds," by which the 

 whole class is frequently designated. Very few other plants vegetate in the sea, 

 seawater being fatal to the life of most seeds ; yet some notable exceptions to this 

 law (in the case of the cocoa nut, 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 Algte. Towai'ds 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 Fuel : and this without reference 

 to the mineral constituents of the rock, the Fucus requiring merely a resting place. 

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

 are, therefore, submerged sandy deserts, as barren as the most cheerless pf the 

 African Avastes. And when such barrens interpose, along a considerable 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 



