on
them to the formation of buds on the leaf of _Bryophyllum_. Others have
been led to see in each placenta, even when it is, to all outward
appearance, a portion of the carpellary leaf, a direct prolongation from
the axis, adherent to the leaf. Teratology shows that ovules may be
formed indifferently on leaf-organs or on stem-organs. Sutural,
parietal, axile, free-central placentation, and, if there be more forms,
all may be met with even in the same ovary (see pp. 96, 508). Now, if
there were such special tendencies in the axis, as contrasted with the
leaf, to produce ovules, it is hardly likely that such anomalous
arrangements as those just mentioned would be as frequent as they are.
But as leaves produce other leaves, from their edges or their surfaces,
and as they form buds in the same situations, just as axial organs
do,[561] there is surely little ground for considering the placentas, or
ovuliferous portions of the plant, to be of necessity axial. Here again,
much of the difficulty vanishes if the morphological identity of the
leaf-form and of the stem-form be admitted.

=Structure of the ovule.=--The nature of the ovule and of its coverings
has been a fertile source of controversy. The teratological data bearing
on this subject have been given at pp. 262-272. These data strongly
support the notion of the foliar nature of the coatings, and of the
axial nature of the nucleus, taking leaf and axis either in the ordinary
sense, or as modifications one of the other. It has been shown that the
ovular coats may themselves become carpels, and that ovules may be
developed upon ovules, p. 268. Whether the intra-carpellary siliques of
_Cheiranthus_, not uncommonly met with (p. 182), are instances of ovular
transmutation may be open to doubt.

The axial nature of the nucleus has been inferred from its position,
mode of growth, and from its occasionally lengthening into a leafy or
even a floriferous shoot. Probably it may occasionally be invested by
sheathing coats, more analogous to tubular processes from the
receptacle, than to foliar organs, as is the case in _Welwitschia_. The
discussion of this matter, however, pertains rather to normal morphology
than to teratology.

=Morphology of conifers.=--The nature of the pseudo-leaves of
_Sciadopitys_, and probably of other Conifers, is illustrated by
teratology, as also is the true constitution of the scale of the cone
(see pp. 192, 245, 352), though it must be admitted that