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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 increas: ing 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 lemma, 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

case of adhesion, that in which the liber, and particularly pith, of two plants unite so nicely together that the part called the graft can receive its sap, and thus live by the nourishment it derives through the organs of the old plant; thus artificially doing what parasitic plants, such as the mistletoe, do by nature. There is, however, a limit to this operation ; if we except parasitical and some few natural adhesions, we shall find that artificially it is only plants of the same natural family that can be grafted together with anything like permanent success, and only those of families strongly analogous in which any union will take place at all. When they are not of the same family the grafts are of short duration in consequence of their physiological difference from the tree to which they have been united. Grafts are of three kinds—that ordinarily so called, in which a severed portion of a stem is united to another tree, whose bark has been cut away at the proper spot, that by approach, which consists of drawing two branches or two trees together, each remaining in the ground held by its own roots, and taking off the bark of each at the point of contact; the liber and pith of the two plants soon unite by the development of their cambium, and one of them may then be cut away below the junction. The third method is by the insertion of a portion of a stem containing a bud in the axil of a leaf, within the bark of the tree on which we desire to ingraft it; the bud thus inserted receives nourishment from the juices of the tree in which it is placed, and is developed as it would have been on the stem from which it was originally taken. 76. There are various subjects of great interest connected with the reproduction of plants, whether from seed or division, but which are too numerous to be dwelt on in an elementary work; among them is the production of hybrid varieties by fertilizing the stigma of one plant with the pollen from another, which may occur accidentally, if the plants are in each other's neighbourhood, or may be effected at pleasure between those whose natural affinities are very close. In this manner modern gardeners have succeeded in raising numberless varieties of favorite genera. The effect of culture and care generally, as is universally known, is to improve the beauty and value of the vegetable productions by which we are so bountifully surrounded. This subject, interesting as it is, can here be only recommended to notice, without further entering on it. Its details may afford to those whose local situation enables them practically to pursue them, an occupation at once healthy to both body and mind, and so connected with chemistry and mineralogy, as to lead on from the simple nurture of a pretty or useful plant, to the study of some of the most important of the sciences. 77. What great antiquity the method of grafting may claim, we may gather from St. Paul's exhortation to the Gentiles in the 11th chapter of his Epistle to the Romans, in which the metaphor is used throughout with an evident knowledge of the subject. Indeed the custom appears to have been one with which practical gardeners have been familiar for ages, and to which attention has been at times particularly turned. In the Philosophical Transactions for 1675, mention is made of a work by Abraham Munting, printed three years before, which shows that his attention was practically given to the cultivation of fruit trees, and to the improvement of the sorts by grafting. “To obtain extraordinary good, large, and beautiful apple fruit,” he advises “by all means to graft good grafts upon such apple stocks as are produced from the seed, and have been deprived of their heart root which shoots downwards.”* To the invaluable and long continued investigations and experiments of Mr. Andrew Knight, however, and to his acute reasoning on the subject, the present highly improved knowledge of the best method of grafting trees, and of the general nature of the subject, is mainly owing. In a paper published in the Phil. Trans. for 1795, Mr. Knight gives a very interesting account of the experiments which convinced him of the fact, so important in its practical results, that “every cutting taken from the apple, and probably every other tree, will be affected by the state of the parent stock. If that be too young to produce fruit, it will grow with vigor but will not blossom ; and if it be too old, it will immediately produce fruit, but will never make a healthy tree, and consequently never answer the intention of the planter.” Having suspected that the decay in some trees he had seen recently grafted might be the consequence of the diseased condition of the grasts, Mr. Knight says, “I concluded that if I took scions or buds from trees grafted in the year preceding, I should succeed in propagating any kind I chose. With this view, I inserted some cuttings of the best wood I could find in the old trees, on young stocks raised from seed. I again inserted grafts and buds taken from these on other young stocks, and wishing to get rid of all connection with the old trees, I repeated this six years; each year taking the young shoots from the trees last grafted. Stocks of different kinds were tried, some were double graf

* Phil. Trans, abridged, vol. xix. p. 192-3, “Account of some new books.”

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