To this formation of adventitious roots the gardener owes the power he has of propagating plants by cuttings, _i.e._, small portions of the stem with a bud or buds attached, or in some cases from portions of the leaves, of the roots themselves, or even of the fruit, as in the case of the cactus (Baillon). Care also has to be exercised in grafting certain fruit trees not to allow the grafted portion to be too close to the ground, else the scion throws out roots into the soil, and the object of the cultivator is defeated.
[Illustration: FIGS. 73 and 74 show formation of roots from leaves induced by the art of the gardener.]
Layering is another garden operation dependent on the formation of these organs, and advantage is also sometimes taken of this tendency of some plants to produce roots when injured to reduce the dimensions of a plant when getting too large for the house in which it is growing. By gradually inducing the production of new roots from the central or upper portions of the stem, it becomes possible, after a time, to sever the connection between the original roots and the upper portion of the trunk, and thus secure a shortened plant.
On the subject of adventitious roots, &c., reference may be made to Trécul, 'Ann. Sc. Nat.,' 1846, t. v, p. 340, et vi, p. 303. Duchartre, 'Elements de Botanique,' p. 219. Lindley, 'Theory and Practice of Horticulture.' Thomson's 'Gardener's Assistant,' pp. 374, _et seq._; and any of the ordinary botanical text-books.
=Formation of adventitious buds on roots.=--One of the characteristics by which roots are distinguished from stems in a general way consists in the absence of buds; but, as is well known, they may be formed on the roots under certain circumstances, and in certain plants, e.g., _Pyrus Japonica_, _Anemone Japonica_, &c. What are termed suckers, owe their origin to buds formed in this situation.
If roots be exposed or injured, they will frequently emit buds. The well-known experiment of Duhamel, in which a willow was placed with the branches in the soil and the roots in the air, and emitted new buds from the latter and new roots from the former, depended on this production of adventitious organs of either kind.
Gardeners often avail themselves of the power that the roots have of producing buds to propagate plants by cuttings of the roots, but in many of these cases the organ "parted" or cut is really an underground stem and not a true root.