71. In whatever manner a seed may be placed in the ground, it invariably shoots forth its plumule in an ascending, and its radicle in a descending direction. Invert it as we may, the result will be the same; but on what vital energy within the plant the constancy of this fact depends, seems yet entirely uncertain. Whether it arise from the tendency of upper portions of plants to seek the light, or from any other cause, the reason is equally obscure, and we can hardly reckon on its being ascertained by the most minute investigation; it seems to belong to that class of phenomena in nature whose ultimate principles are too subtle for our grasp, and appear to depend on that vitality which we can indeed perceive most palpably in its effects, but whose cause is known only to the Creator; whether modern science will be permitted to approximate nearer to the truth on this and some few similar subjects must remain at least doubtful: at all events we are not now in possession of any wholly satisfactory solution of the difficulty.

"That gravity is an important agent in determining the difference between the directions taken by the root and stem, is shown by an ingenious experiment of Mr. Knight. He placed some French beans on the circumference of two wheels, and so secured them that they could not be thrown off when a rapid rotatory motion was given to the wheels. One wheel was disposed horizontally, the other vertically, and both were kept in constant motion while the beans were germinating. The radicles of those beans which germinated on the vertical wheel extended themselves outwards, or from the centre, and the plumules inwards, or towards it. Those which were placed on the horizontal wheel pushed their radicles downwards and their plumules upwards;

but the former were also inclined from and the latter towards the axis of the wheel. This inclination was found to be greater as the velocity of the wheel was increased. Now in the vertical wheel the effects of gravity were nullified; since the beans were constantly changing their position with respect to those parts which were alternately uppermost and lowermost, in each revolution. The only cause which could have produced the effects described must be the centrifugal force, which has here replaced the effects of gravity, compelling the root to grow outwards and the stem inwards, instead of downwards and upwards. The effect produced upon the horizontal wheel is evidently the result of the combined action of the forces-gravity inclining the root downwards, and the centrifugal force propelling it outwards; and the reverse with regard to the stem. Although it is plain that gravity is the efficient cause in establishing the directions of the stems and roots of plants, it is not so easy to understand the manner in which it produces opposite effects on these two organs. Various theories have been formed to account for this, and the most plausible is that which ascribes it to the different manners in which the newly developed tissues are added to the root and stem. In the root the addition is almost entirely confined to the very extremity, while the stem continues to increase for some time through its whole length. Hence it is supposed that the soft materials continually deposited at the extremity of the root must ever be tending downwards from the effect of gravity alone." (Henslow's Principles of Botany, p. 292.)

Is it not probable that we may find the agency of light connected with the fact of the plumule ascending?

72. The reproduction of the tribes of the Cryptogamia takes place in a very different manner from that of the flowering plants. In all of them it occurs spontaneously, and without any contact between one part of the plant and another. At the season of the year when the lowest tribes of all, such as the Red Snow, the Confervæ, &c., are to reproduce their species, a number of small granules are liberated by the bursting asunder of the cell which enclosed them. They gradually develop themselves into cells, acquire the size and form of the parent plant, and become distinct individuals, capable in their turn of producing others like themselves. The apparatus of reproduction if we may so call it, increases in complexity as it approaches the higher orders, but in all except the cells just mentioned, the immediate organ is called a spore,* and is analogous to the seed of the flowering plant.

73. It has been seen that in reproduction by seed, each germ has the power of becoming developed, after fecundation, into a separate individual plant, entirely distinct from that which gave it birth. In addition to this accustomed mode of increase, plants are also propagated by division; and this is either natural or artificial, and depends on two circumstances: in one, the ascending organs are first developed, or in other words an adventitious leaf bud (35) is produced, and these favor the subsequent development of the roots; in the other, roots are first formed, and by their action promote the development of the

* "It is in the spores that the power of increase resides; every one of them will form a new plant, and consequently they are analogous to seeds, but, as they do not result from the action of pollen upon a stigma, they are not real seeds, but only the representations of those organs amongst the flowerless plants." (Lindley's Ladies' Bot., p. 270.)

ascending system. The former is in general the case when the germ is found surrounded by a sufficient deposit of nourishment to sustain it till it can push forth its roots: this nourishment is furnished by the mother-plant from the descending juices. To this sort of buds may be given the general name of tubercles, though botanists designate them by a variety of appellations. In all tubercles a phenomenon occurs which distinguishes their germination from that of seed; in the latter the radicle is always first developed, while in the tubercle the ascending part, -that which corresponds to the plumule,-is first put forth. The common potato is an instance of this mode of increase; the tubercles are detached towards the end of the year either by the death of the stem on which they grow, or by the slightest accident, and falling on the ground, vegetation ensues. This single example is sufficient for the present purpose; the phenomenon exists in many other plants under various forms. In the cases in which vegetation commences in the descending system, that is, in which roots, whose development is always effected through the descending juices, are first formed, the result is produced in some portion of the stem which is found to contain a deposit of nutritive matter, and which is within reach of moisture. This effect occurs naturally in some stems, but is facilitated by any cause which tends to arrest the nutritive juice in its descent, and so to form an accumulation of it at a given part. Thus in nature when a portion of a stem containing such an accumulation, is buried beneath a humid soil, and has a fleshy bark, it tends to put out roots, which it does naturally by what are called "suckers," and man, profiting by this provision, adopts the method of increasing by layers, pipings, cuttings, &c., since it is found

that the part thus endowed may be separated from the parent trunk, and being composed of the two parts that constitute an individual plant, a stem and a root, is capable of an independent existence. In some instances a leaf planted in the ground will vegetate from its central nervure.

74. There is one great difference notwithstanding so much apparent identity, between the products of the two methods of reproduction above mentioned. In the case of propagation by seed, the embryo is really, and from the first moment of its existence, a being distinct from the parent plant, the seed is furnished with all the organs it requires; the tubercle, on the contrary, is but a fragment of the plant that bore it, and has gradually to form for itself the needful organs. The seed, being entirely distinct, may only resemble the original plant by the general characteristics that belong to its kind; while the tubercle or the cutting, being actually portions of the plant itself, preserves its minutest particularities. A very curious instance of reproduction occurs in the lemna, or common duckweed. If one of its little discs be placed in a saucer, we shall soon see it send forth laterally a tubercle which grows in a horizontal direction, puts out a root underneath, and thus forms a second plant similar to the former, but united with it. This double disc continues to vegetate in the same manner, and so on.

75. Besides the method of increase by cuttings, tubercles, &c., mentioned above, another exists which is, as all gardeners well know, of immense practical utility that of grafting. All parts of plants have the power of uniting together by their cellular tissue, -thus we see even in those which consist only of cellular substance that such adhesions take place. The name of graft has been especially given to one