his root-like structure was found in the centre and
attached to one end of the cavity.

The production of roots which ultimately serve as props to support the
branches, or as buttresses to compensate for the increasing weight of
branches and foliage, is also a familiar occurrence. The huge gnaurs and
burrs met with occasionally on some trees often produce great
quantities, not only of adventitious buds, but of roots also.

[Illustration: FIG. 71.--Production of adventitious roots from leaf
stalk of celery.]

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 t