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WHOEVER has paid the slightest attention to the classification of natural objects, whether plants or animals, must be aware, that if we desire to follow natural principles in forming our groups,—that is, to bring together such species as resemble each other in habit, properties and structure,-it is a vain task to attempt to define, with absolute strictness, the classes into which we are forced to combine them. At least, no effort to effect this desirable object has yet been successful. Natural groups are so interwoven into each other, and often exhibit such an exaltation and degradation of characters within the limits of an Order or a Genus, that the distinctive marks, as they approach each other, gradually disappear, and two tribes, which in the more highly developed species, scarcely resemble each other, are found, in the lower, to be either undistinguishable or with difficulty distinguished. Thus, to a common observer, the Poppy and the Fumitory would scarcely be supposed to be closely related; yet there is such a perfect gradation between them, through allied genera, that they are now generally referred to the same natural order. Still more unlike in appearance are the Rose and the Shamrock, yet they belong to orders so closely connected, that the only invariable mark by which they can be distinguished is, that in one, what is called the odd segment of the calyx is posterior, while in the other it is anterior; and till this was pointed out by Mr. R. Brown, botanists were at a loss to define the respective orders, though very seldom indeed puzzled as to whether a genus were Rosaceous or Leguminous. If it be thus difficult to define groups among highly organized plants, it can be no matter of wonder that when we come to the CRYPTOGAMIA, whose structure is so much more simple and uniform, and whose forms are still more sportive, the difficulties become vastly increased. But it fortunately happens that these difficulties are much more formidable on paper than in the field. Thus, while the systematizer, in his study, may consume the midnight oil till his aching brains are weary with the fruitless task, in attempting to express in words a character which shall include every species of the class AlgÆ, and, at the same time, exclude every denizen of the allied groups, FUNGI and LICHENES; the student, roaming through the fields or along the sea-shore, finds no difficulty whatever in recognising a sea-weed as distinct from a mushroom or a Lichen. The search into structure and affinities among the works of creation is something like that after first principles. We can distinguish and analyse up to a certain point: there we are stopped by that invisible and intangible, but impassable veil, in which the Creator hides his operations. At this point we must rest satisfied with differences which we can see, but which we cannot know or define. Dismissing therefore, speculations on the exact limits between ALGÆ and all other tribes, let us proceed to consider the subject more immediately before us, namely, the habit, structure, geographical distribution and uses of these plants.
The name ALGÆ is assigned by botanists to a large group or natural class of Cryptogamic or flowerless plants, which form the principal and characteristic vegetation of the waters. The sea, in no climate from the poles to the equator, is altogether free from them, though they abound on some shores much more than on others, a subject which will come particularly under notice when we speak of the distribution of their several tribes. Species abound likewise in fresh water, whether running or stagnant, and in mineral springs. The strongly impregnated sulphureous streams of Italy,—the eternal snows of the Alps and arctic regions, --and the boiling springs of Iceland, have each their peculiar species; and even chemical solutions, if long kept, produce Algæ. Very few, comparatively, inhabit stations which are not submerged or exposed to the constant dripping of water; and, in all situations where they are found, great dampness, at least, is necessary to their production.
Thus extensively scattered through all climates, and existing under so many varieties of situation, the species are, as one would naturally suppose, exceedingly numerous, and present a greater variety in form and size than is observable in any other tribe of plants whose structure is so similar. Some are so exceedingly minute as to be wholly invisible, except in masses, to the naked eye, and require the highest powers of our microscopes to ascertain their form or structure. Others, growing in the depths of the great Pacific Ocean, have stems which exceed in length (though not in diameter) the trunks of the tallest forest trees; and others have leaves that rival in expansion those of the Palm. Some are simple globules or spheres, consisting of a single cellule or little bag of tissue filled with a colouring matter; some are mere strings of such cellules cohering by the ends; others, a little more perfect, exhibit the appearance of branched threads; in others, again, the branches and stems are compound, consisting of several such threads joined together; and, in others, the tissue expands into broad flat fronds. Only the higher tribes show any distinction into stems and leaves, and even in these, what
appears a stem in the old plant, has already served at an earlier period of growth, either as a leaf, as in Sargassum and Cystoseira, or as the midrib of a leaf, as in Delesseria. A few exhibit leaves or flat fronds formed of a delicate, perforated net-work, resembling fine lace or the skeletons of leaves, a structure which is also found among zoophytes. Of those so constructed the most remarkable are the New Holland genus, Claudea, the East Indian Dictyurus (Callidictyon, Grev.) and a genus still unnamed, lately discovered at Port Natal, in South Africa, by Dr. Krauss, which produces fan-shaped fronds, the lower half of which has the structure and colour of Nitophyllum, the upper that of the delicate net-work of Claudea. Among British Algæ, the only structure analogous to these exists in Hydrodictyon, which grows in the form of a perfect net, with regular meshes.
The substance of which the frond consists is as variable as the form. Some are mere masses of slime or jelly, so loose that they fall to pieces on being removed from the water; others resemble, in feel and appearance, threads of silk; some are stiff and horny; others are cartilaginous, or with the aspect and elasticity of gristle; others tough and coriaceous, or resembling leather; while the stems of some of the larger kinds are almost woody. The leaves of some are delicately membranaceous, glossy and transparent; of others, coarse and thick, and either wholly destitute of nerves, or furnished with more or less defined ribs; or beautifully veined. Among the most minute kinds, many (comprising the family Diatomacee) are cased with organized silex, and these cases, which resist the action of fire, are found, in countless myriads in a fossil state, in many countries, covering miles of ground, or forming mountains, and presenting to the naked eye a whitish, powdery substance, known by the name of "mountain meal."*
* See Ehrenberg's discoveries.
In colour, the Algæ exhibit three principal varieties, with, of course, numerous intermediate shades, namely, grass-green, olivaceous, red. The grass-green is characteristic of those found in fresh water, or in very shallow parts of the sea, along the shores, and generally above half-tide level; and is rarely seen in those which grow at any great depth. But to this rule there are exceptions, sufficiently numerous to forbid our assigning the prevalence of this colour altogether to shallowness of water. Several of the more perfect Confervee and Siphonee grow beyond the reach of ordinary tides; and others, as the beautiful Anadyomene, are sometimes dredged from very considerable depths. The great mass, however, of the green-coloured species, are inconsiderably submerged. The olivaceous-brown or olivegreen is almost entirely confined to marine species, and is, in the main, characteristic of those that grow at half-tide level, becoming less frequent towards low-water mark; but it frequently occurs also at greater depths, in which case it is very dark, and passes into brown or almost black. The red also, is almost exclusively marine, and reaches its maximum in deep water. When it occurs above halftide level it assumes either purple, or orange, or yellow tints, and sometimes even a cast of green, but in these cases it is sometimes brightened by placing the specimens, for a short time, in fresh water. It is rarely very pure much within the range of extreme low-water mark, higher than which many of the more delicate species will not vegetate; and those that do exist degenerate in form as well as in colour, as they recede from it. How far below lowwater mark the red species extend has not been ascertained, but those from the extreme depths of the sea are of the olive series in its darkest form. For the colours of these last it has puzzled botanists not a little to account. It is well known that light is absolutely necessary to the growth of land-plants, and that the green colour of their