The third class of plants whose roots are perennial, or live an indefinite number of years, is the largest of the three grand divisions. It comprises all our forest and shade trees, and most of our beautiful flowering plants, and their forms are no less desirable than in the foregoing grand divisions. The most common form of the perennial root is the branching. The roots resemble the branches of a tree, but have no regularity. Some of these branches penetrate the earth to a great depth. Some of them trail horizontally beneath the surface. They do not, as the foregoing, depend upon the stock of the previous year, but annually produce new roots, and form new accumulations, sometimes in separate portions of the root, as in the dahlia or in the Orchis, where, while one or more of such reservoirs is exhausted each year, others are providentially formed for the next years sustenance; and so on from year to year, a portion annually perishing, but the individual plant surviving indefinitely. More commonly, the whole body and main branches of the root are somewhat thickened, or portions of the stem may subserve this purpose, as in all tuberous roots; or the nourishing matter may be widely distributed through the trunk, as in shrubs and trees.

Thus far we have only considered the primary root, or that organ which originated in the cmbyro of the seed at germination. Adventitious buds grow from most stems when placed horizontally on the ground, or buried beneath it, and this is very common to some species; thus the verbena is furnished with joints at intervals of several inches each, all of which strike when placed in contact with the soil. "Such roots obey the ordinary tendency of the organ, avoiding the light, and seeking to bury themselves in the soil. Thus when a part of the stem of a plant be cut off at each end, and subjected to the requisite amount of light, heat, and moisture - they will* in nine cases out of ten take root. It is in this manner that most green-house shrubs and plants are cultivated. Many plants will emit roots from their leaves. For instance, if the leaf of a Gloxinia or Gesneria be pinned upon the surface of some soil placed in a pot, roots will be seen to issue from the stem in the course of from four to six weeks." To this, as in every other general rule, there are exceptions - as in the case of atrial roots.

Such plants as the Poison Ivy, (Rhus toxicodendron,) Trumpet flower, (Tecomia radicans,)etc, emit aerial rootlets from their stems, which fix them to any object which may present itself; in this case the plant draws its nourishment from the soil through its roots, the aerial ones only serving for mechanical support; in this way a plant, in the course of a few years, reaches the tops of the highest trees. There are other plants which produce true aerial roots, which are emitted from the stem, descend to the ground and grow. This phenomena may be observed in our common Indian corn, the lower joints of which often send down roots the length of three and four inches; but is more perceptible in the vegetation of a southern climate, where the atmosphere is charged with moisture for a large part of the year. The Pandanus or Screw Pine, forms a good illustration. The roots are emitted from the stem, and fall down, which gives the tree the appearance of having been raised out of the ground. The Banyan also affords another illustration; in this case the roots spring from the horizontal branches, often at a great length from the ground; they then descend, take root, and produce in their turn, plants similar to themselves.

This tree grows to an immense size; there is one in the world, which, it is said, is sufficiently large to shelter ten thousand men. The Mangrove is very similar in its habits.

And from the branches, as in the Banyan. Moreover, this tendency to shoot in the air is shown even in the embryo, which begins to germinate while the pod is yet attached to the-parent branch; the radicle, or root end of the embryo, elongating into a slender thread, which often reaches the ground at the height of many yards, before the pod is detached. In this manner the Mangrove forms those immense maritime thickets which abound on low muddy shores within the tropics. There is a class of plants called Epiphytus or air plants, which exhibit a further peculiarity. They not only emit roots from every part of their trunks, but during their whole life have no connection with the soil; they are generally found growing upon bark, and the trunks of old trees. The roots adhere to the bark, and fix the plant in a steady position, or else hang loose in the air, from which such plants draw all their nourishment. The parasites are mostly natives of southern regions, such as the orchidaceous plants; many of them adorn our hot-houses, and are rare and interesting objects. Some parasites not only grow upon other plants, but live wholly at their expense, which the epiphytes do not.

Parasites may be reduced to two different sorts: first, green parasites, those which have green and proper foliage for respiration and perspiration - and second, those which are destitute of green foliage; they also differ in their degree of parasitism - the great number of them being dependant upon the foster plant for support; but there are a few, such as the Colutiarosea, which often take root in the soil, and from thence assimilate a part of their food, and in some cases live and grow in* dependent of their aerial roots. The green parasites are furnished with proper digestive organs of their own, just as in the higher class of flowering plants; they strike their aerial roots through the bark of the plant upon which they grow, and embed themselves in the alburnum, from which they can draw little or no sustenance, except the crude ascending sap, which they must assimilate with their own organs. The Misletoe is always parasitic, being at no time connected with the soil; the seed germinates upon the tree wherever it happens to fall; the germinating root, or the woody mass which it forms resembling the root, penetrates the bark of the foster plant, and forms a close junction apparently, with its young wood, as that of a natural branch.

Some species of the Misletoe have no proper green colored foliage, but are of a brown or yellow cast. Pale or colored parasites, such as the Beech drops, strike their roots in the bark of the foster plant, and thence draw their nourishment, already assimilated. Hence they have no use for their proper colored foliage. In some instances, such plants as the Dodder will germinate in the earth, but as soon as they grow large enough they twine around some approximate tree, their aerial rootlets penetrating the epidermis into the bark, and feed upon its nourishment - while its own root dies, and the plant has never any more connection with the soil; thus the plant, like some human ones, steals its nourishment, and requires no proper foliage, for it would not use it if it had it. Such parasites do not live upon all plants, but only upon those which will yield a propitious food. Some, it is said, are restricted to certain species, and others seem to have little or no choice. Their seeds are only germinated when placed in contact with the plant upon which they are to grow. Some parasites may be reduced to a single flower, or flowers, situated immediately upon the foster plant.

A truly wonderful instance of this kind is furnished by that vegetable titan, the Rafflesia arnoldi, of Summatra. The flower which was first discovered grew upon the stem of a kind of grape-vine; it measured nine feet in circumference, and weighed fifteen pounds. Its color is of light orange, mottled with yellowish white. Some cryptogamous plants such as the fungi are parasi