Very frequently the spherical vesicles, which have al ready been compared to juniper berries, are as large as small purple grapes, and have a striking resemblance to them. For this reason, the name of Tropic Grape is often applied to the floating sea-weed. As a proof that the ve sicles are intended, in the case of F. natans, chiefly to give buoyancy to the plant, Ruiz states that, when the vesicles are all cut off, the plant sinks.
The great collections of floating sea-weed which have now been described are not without their use in the eco nomy of nature ; for they afford both food and shelter to glyriadF of fishes and molluscs, and probably tend, by giv ing forth oxygen, to maintain the wholesome purity of the sea. To the mariner, the young or most succulent shoots of F. ratans offer an acceptable salad, or they arc prepared as a pickle like samphire.
Economical Uses of Sca•weeds It is observable on most coasts, that sca-weeds, or many species at least, very suddenly disappear from the rocks in the autumn. They do not decay, like land plants, on the spot where they grow ; but, losing their hold, are washed away by the tides ; and, in the narrow seas at least, gene. rally wafted ashore, to offer their services to man. Among the Romans, indeed, they were proverbially useless. When they wished to stigmatise any thing as utterly worthless, it was declared to be algd projectd vilior ; and Horace, when he speaks of alga covering the shores as drift-ware, thrusts in the epithet inutilis. In modern times, the alga marina has become useful and valuable in various respects. To the agriculturist it furnishes a most important manure. To the glass-maker and soap-boiler it yields the fixed alkali ; and the manufacture of kelp for this purpose, has become a valuable source of revenue to the proprietors of the rocky shores of Europe, particularly of Britain, and more espe cially to those of the Northern and Western Islands of Scotland. Of such importance has this manufacture ap peared, that in some places attempts have even been made, and not without success, to cultivate the fuci. By merely covering sandy bays with large boulder stones, a crop of fuel has been procured in the course of two or three years, the sea appearing every where to abound with the neces sary seeds. From the ashes of the fuci the chemist has of late years derived the very curious elementary substance named iodine. Several of them are so rich in saccharine matter, and vegetable mucilage, that on the shores of the northern countries of Europe, and in the Scottish islands, much of the winter provender of cattle is derived front them ; and in the city of Edinburgh these plants are occa sionally given as a useful stimulus to the stomach of mulch cows kept in confinement during the winter. A few of
them even afford food to man. What might least of all be expected, two or three of them furnish fuel to the inhabi tants of coasts where materials for firing are scarce. Some of the smaller sorts yield various condiments, or aflOrd fresh salads ; while others are employed as medicines. Front a few of them, substances useful in the arts are pro cured; and with some of the more delicate and elegant spe cies, ornamental pictures are constructed.
While considering the different purposes to which ma rine plants may thus be applied, it may be agreeable to the reader to see the descriptions of those chiefly employed, and more especially of such useful species as inhabit the British shores. The descriptions, however, must consist chiefly of the very accurate specific characters drawn up by Mr 'Curlier, to which some explanations shall be added, where they seem requisite.
It is scarcely necessary to say, that every kind of sea weed may be employed as manure. In point of fact, what is used for this purpose is that which is cast ashore by storms in the winter months, consisting of all sorts, mixed with zoophytes, and all the other rejectamenta of the sea. In many places, the value of such manure is -duly appre ciated ; while in others it is unaccountably neglected ; not that it is any where entirely despised, but it frequently hap pens that on one day many tons of drift-ware are cast on a particular shore, and that on the next the whole is swept away. They who would avail themselves of this bounty of the deep, must snatch the moment of its beiDg, placed with in their power, and muster all hands to drag it at least be yond the reach of the returning tide. It must nut be left very long in the heap, nor suffered to ruo into the putre factive fermentation ; for in this case, suiplim eted hydro gen, and other gases highly important in promoting vege tation, escape rapidly, and in gi eat quantity ; leaving com paratively inert mass. But for details as to the mode of applying this manure to lands, so as best to secure its fer tilizing effects, reference may be had to the article AGRI CULTURE.