of the rose which bears opposite leaves, in that of the
Imatophyllum, &c., there is no known natural species or seedling variety,
from which the characters in question could have been derived by crossing.
We must attribute all such cases to actual variability in the buds. The
varieties which have thus arisen cannot be distinguished by any external
character from seedlings; this is notoriously the case with the varieties
of the Rose, Azalea, and many other plants. It deserves notice that all the
plants which have yielded bud-variations have likewise varied greatly by
seed.

These plants belong to so many orders that we may infer that almost every
plant would be liable to bud-variation if placed under the proper exciting
conditions. These conditions, as far as we can judge, mainly depend on
long-continued and high cultivation; for almost all the plants in the
foregoing lists are perennials, and have been largely propagated in many
soils and under different climates, by cuttings, offsets, bulbs, tubers,
and especially by budding or grafting. The instances of annuals varying by
buds, or producing on the same plant {408} differently coloured flowers,
are comparatively rare: Hopkirk[952] has seen this with _Convolvulus
tricolor_; and it is not rare with the Balsam and annual Delphinium.
According to Sir R. Schomburgk, plants from the warmer temperate regions,
when cultivated under the hot climate of St. Domingo, are eminently liable
to bud-variation; but change of climate is by no means a necessary
contingent, as we see with the gooseberry, currant, and some others. Plants
living under their natural conditions are very rarely subject to
bud-variation: variegated and coloured leaves have, however, been
occasionally observed; and I have given an instance of the variation of
buds on an ash-tree; but it is doubtful whether any tree planted in
ornamental grounds can be considered as living under strictly natural
conditions. Gaertner has seen white and dark-red flowers produced from the
same root of the wild _Achillea millefolium_; and Prof. Caspary has seen
_Viola lutea_, in a completely wild condition, bearing flowers of different
colours and sizes.[953]

As wild plants are so rarely liable to bud-variation, whilst highly
cultivated plants long propagated by artificial means have yielded by this
form of reproduction many varieties, we are led through a series such as
the following,--namely, all the eyes in the same tuber of the p