The leaves, equally with the stems, have the power of emitting roots under certain conditions, as when the leaves are in close contact with moist soil or as the result of injury. This happens in some plants more readily than in others--_Bryophyllum calycinum_ is a well-known instance. Mr. Berkeley has described the formation of roots from the fractured leaves of celery,[147] and also in a cabbage where a snail "having gnawed a hole into the middle of a leaf at its junction with the stem, a fascicle of roots was formed, bursting through the tissue lining the cavity, and covered with abundant delicate hairs after the fashion of ordinary radicles." [Illustration: FIG. 72.--Germinating plant of mango, showing production of roots from one of the cotyledons (from the Kew Museum).] The production of adventitious roots is not limited to the ordinary leaves of the plant, but may be manifested on the cotyledons; thus Irmisch describes cases of this kind in the cotyledons of _Bunium creticum_ and _Carum Bulbocastanum_.[148] I have figured and described an analogous case in the cotyledons of the Mango (fig. 72).[149] 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 Trecul, '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.