nce that
needs especial mention under this section is the great lengthening that
sometimes takes place in the carpels, sometimes as a result of injury
from insects or fungus, at other times without assignable cause.

[Illustration: FIG. 208.--Leaves of horse-chestnut, _AEsculus_, showing
passage from digitate to pinnate leaves.]

In the case of inferior ovaries this lengthening is, perhaps, even more
common, as in _Umbelliferae_, _Compositae_, &c. The common groundsel
(_Senecio vulgaris_) is especially liable to this form of enlargement of
the pistil, either in association with a leafy condition of the pappus
or without any such change.

=Elongation of the thalamus, placenta, &c.=--In some plants, as in
_Magnolia_ or _Myosurus_, the thalamus becomes much elongated, and bears
the carpels disposed spirally around it. A similar lengthening occurs in
malformed flowers, usually in association with a similar change in the
lower or outer part of the flower, by virtue of which the whorls become
separated from each other (Apostasis). Elongation and protrusion of the
placenta have been already alluded to at p. 119, and also at p. 125. In
some of these cases the elongated placenta has taken the form of a
leaf-bearing shoot.[515]

=Apostasis.=--Engelmann made use of this term to express the separation
of parts one from another by the unusual elongation of the
internodes.[516] He drew a distinction between the separation of
individual organs one from the other, and the corresponding displacement
of whorls. The subject has already been, to a considerable degree,
treated of in these pages under the head of dialysis, displacement, and
prolification, and but little need here be added. With reference to the
distance between one whorl and another, it will be remembered that,
although in the majority of cases the floral whorls are packed closely
together, yet in other instances the floral axis becomes elongated, and
thus separates the whorls one from another, by structures such as the
gynophores, androphores, &c., of _Passifloreae_, _Caryophylleae_,
_Capparideae_, &c. &c.

A similar elongation of the thalamus, bringing about the separation of
the floral whorls, or of their constituent parts, is very commonly met
with in association with median prolification. Where the individual
floral elements are thus thrown out of their usual verticillate
arrangement, they naturally assume a spiral disposition, and are, in
some cases, united by th