HANDBOOK OF SEA-WEEDS. 5 



i 



jelly, and the restorative value of which in consumption doubtless 

 depends in some degree on the presence of iodine. The freshwater 

 Algoe not only furnisli abundant and nourishing food to the fish 

 and other animals living in ponds and streams, but by their 

 action in the decomposition of carburetted hydrogen and other 

 noxious gases purify the element in which they live, thus be 

 coming important sanitary agents. The value of aquatic plants 

 in the aquarium is well known. A Chinese species of Gigamna 

 is much employed as a glue and varnish ; and also much used in 

 China in the manufacture of lanterns and transparencies, and in 

 that country and Japan for glazing windows. Handles for 

 table knives and forks, tools, and other implements have been 

 made from the thick stems of oarweeds, and fishing lines from 

 Chorda filum. Tripoli powder, extensively used for polishing, 

 consists mainly of the silicious shells of Diatoms. On various 

 parts of our coast, the coarser species of sea-weed, now used as 

 a valuable manure, were formerly extensively burnt for kelp, 

 an impure carbonate of soda. This industry, when carried on 

 upon a large scale, became a fruitful source of income to some 

 of the poorest districts in the kingdom, bringing, in the last 

 decade of last century, nearly ^"30,000 per annum into Orkney 

 alone. Since the production of soda from rock salt has become 

 general, kelp is now only burnt for the extraction of iodine, this 

 being the easiest way of obtaining that substance. 



Although the vegetable structure and mode of reproduction 

 are essentially the same in all Algoe, as regards the former they 

 vary from the simple cell, through cells arranged in threads, 

 to a stem and leaves simulating the vegetation of higher 

 tribes. And although the simpler kinds are obviously formed 

 of threads, most of the more compound may also be resolved 

 into the same structure by maceration in hot water or diluted 

 muriatic acid. In substance some are mere masses of slime or 

 jelly, others are silky to the feel, horny, cartilaginous or leather- 

 like, and even apparently woody. A few species secrete car- 

 bonate of lime from the water, laying it up in their tissues ; 

 others cover themselves completely with that mineral, while 

 some coat themselves with silex or flint. Many Algse are 

 beautifully coloured, even when growing at depths to which 

 very little light penetrates. As in their vegetative organs, so in 

 their reproductive, Algae, exhibit many modifications of struc- 

 ture without much real difference. In the green sea-weeds 

 reproduction is effected by simple cell division in the unicel- 

 lular species, and by spores resulting from the union of the 

 contents of two cells in the others. The red sea-weeds have a 



