Many foci, such as the common F. vesiculosus and se• tatos, have small tufts of white filaments like hairs scatter ed over their surface. Reaumur conjectured, that these might be a sort of male flowers, and as such he described them. In this notion, he was followed by some distinguish ed botanists. But they arc now known to he analogous merely to the hairs on land plants, and to be useful proba bly for the secretion or absorption of particular fluids. It has generally been said, that they occur on the plant in every stage of its growth, and at all seasons of the year. lAmouroux, however, asserts, that they are not permanent, but disappear at certain seasons, and at certain stages of the life of the plant. They are never seen on the stern or the nerve, but on the frond only.
It does not appear probable that the submersed algae derive much of their nourishment from the processes by which they are attached to rocks or other substances, although these processes in many instances resemble roots, and usually receive that name. It appears likely, that the principal part of their food is imbibed by their general surface. They are attached to rocks of mica-slate, greenstone, basalt, sandstone, and limestone. Many of the smaller kinds grow on the stems of the larger plants. It is to be observed, however, that Mr Turner, in the intro duction to his Synopsis of British Fuci, hints it as his opi nion, that the rout-like processes are not merely intended for fixing the plants, but arc to a certain degree useful as organs of nutrition ; and he mentions that some species seem peculiar to chalk, some to sandstone, and others to still harder rocks. On the other hand, at least one species, Focus bacciferus of Turner, (t. 97), has never been found with a root or base : it evidently lives and increases while; floating about. The extensive meadows of sea-weeds, through which navigators to distant countries continue to sail for days in the unfathomable ocean, and which arc found in a fresh and growing state, afford convincing evi dence that certain kinds of fuci at least are not entirely, nor even chiefly, dependant on thei• roots for nourishment. M. Desvaux, editor of the Journal de Botanigue, has lately made a direct experiment on this subject. Having detach ed some foci above the roots, he fixed them to stones by means of cords or other artificial methods, and plunged them again in the sea. Having visited them some time af
ter, he found that they had increased very sensibly.
Some authors have supposed that the Cuci are nourished by.their mucilage : but this is only removing the difficulty one step; and besides, particular species, as F. esculentus, possess little or no mucilage. It has been remarked, how ever, that the cups or discs of F. loreus become flaccid and destitute of juice, whenever the thongs of the plant have attained full size; and this has been ascribed to the exhaustion of the mucilage by their growth.
The Caulerpx of Lamouroux creep in the sand, and seem to possess true roots; but these, though at present associated with the foci, form not only a very distinct ge nus, but a separate family.
Colours of Fuci.
The colours of sea weeds have not yet been much at tended to. A great proportion of the fuci are of an olive hue ; olive brown, olive yellow, or olive green. A good many are red or brownish red. Some are verdigris green, particularly F. aeruginosus, (Turn. t. 147.) a species from the Red Sea, resembling Lichen prunastri, and F. Valen tine, (t. 78), brought also from the Red Sea, by the noble man whose name it hears ; the latter rendered more re markable by producing bright red spherical tubercles, which form a striking contrast with the glaucous frond.
Some phxnomena not uninteresting to the physiological chemist are slightly mentioned by Professor 'Mertens in the .4llgemeine literatur Zeitung for 1810, in his critique on Mr Turnei 's History of Fuci. He states that F. ligulatus and F. viridis (Turn. t. 97.) possess the property of suddenly alter ing the colour of other foci along with which they happen to he steeped in a vessel full of fresh water ; the red colour of Conferva rubra, or Focus oxalis, being changed to pur ple, and these plants rendered at the same time much more prone to decomposition, while the ligulatus and viridis re main unhurt. F. viridis itself undergoes remarkable changes. While the plant is growing, it is of a• deep rich orange ; but, says Mr Turner, " no botanist can have gathered it without being struck by the circumstance, that before he had conveyed it home, it had changed its origi nal golden hue to a bright verdigris green." At the same time, it undergoes a change in its substance ; from being stiff and elastic, it becomes completely flaccid.