IV. 
INTRODUCTION. 
19 
ancl 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 the 
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 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 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 
paper, which has also been described under the name of water flannel , sometimes 
covers immense tracts, limited only by the extent of the flood in whose waters it 
originated. 
But though Algae 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 Algrn. 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. 
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 of the 
African wastes. 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 
