cility with which axile
placentation becomes parietal, the change being here effected by the
prolongation of the axis, and the formation on it of a second whorl of
carpellary leaves.

In double flowers of _Primulaceae_ similar alterations in the
placentation may often be observed. I have seen in _Primula sinensis_
sutural, parietal, axile, and free central placentation all on the same
plant; nay, even in the same capsule the ovules may be attached in
various ways, and transitions from one form of placentation to another
are not infrequent. The late Professor E. Forbes describes[103] an
instance of true foliar and true axile placentation in the same flower
in _Vinca minor_.

These and many similar changes, which it is not necessary further to
allude to, are not so much to be wondered at when it is borne in mind
how slight an alteration suffices to produce a change in the mode of
placentation, and how frequent is the production of adventitious buds or
of foliar outgrowths, as may be seen in the sections relating to those
subjects and to Substitutions.

It will be remembered, also, how, in certain natural orders, under
ordinary circumstances, considerable diversity in placentation exists,
according as the margins of the carpels are merely valvate or are
infolded so as to reach the centre. Often this diversity is due merely
to the changes that take place during growth; thus, the placentation of
_Caryophylleae_, _Cucurbitaceae_, _Papaveraceae_, and many other orders,
varies according to the age of the carpel, and if any stasis or arrest
of development occurs the placentation becomes altered accordingly.

It is not necessary, in this place, to enter into the question whether
the placenta is, in all cases whatsoever, a dependence of the axis, as
Payer, Schleiden, and others, have maintained, or whether it be foliar
in some cases, axial in others. This question must be decided by the
organogenists; teratologically, however, there can be no doubt that
ovules may be formed from both foliar and axial organs, and, moreover,
that, owing to the variability above referred to, both in what are
called natural and in what are deemed abnormal conditions, it can rarely
happen that any safe inferences as to the normal or typical placentation
of any family of plants can be drawn from exceptional or monstrous
formations.

On the subject of placentation the following authors may be consulted:

R. Brown, 'Ann. Nat. Hist.,' 1843, vol