palustre_.[513] In _Compositae_, when affected by an
analogous change, the capitulum assumes the appearance of a simple
umbel, as in _Hypochaeris radicata_, _Senecio vulgaris_, and other
plants.

[Illustration: FIG. 206.--Inflorescence of _Ranunculus acris_, with
secondary peduncles lengthened.]

In some of the double-flowered apples which have been previously alluded
to, the flower-stalk is inordinately long when compared with the
adjacent ones. Possibly in some of these cases the absence of the usual
swelling of the upper part of the peduncle may be connected with its
increased length. One of the most striking instances of lengthened
flower-stalk occurred in an apple flower, wherein there was no swelling
beneath the calyx, while the latter was represented by five perfect
stalked leaves.

=Elongation of the leaves.=--In the case of water plants this change
keeps pace with the corresponding growth of the stem, _e.g._
_Ranunculus fluitans_, and in terrestrial plants there are varieties
termed longifoliar, from the unusual length of the leaves. A similar
lengthening occurs in the involucral leaves of _Umbelliferae_ and
_Compositae_, changing very materially the general aspect of the
inflorescence. Occasionally, also, the leaf-lobes of parsley (_Apium
Petroselinum_) and other crested-leaved plants may be observed to lose
their ordinary wavy form, and to be lengthened into flat riband-like
segments, as shown in fig. 207.

The only further illustrations that it is requisite to give of such
changes in this place are those occurring in lobed or compounded leaves,
which, from a lengthening of the midrib or central stalk, convert a
digitate or palmate leaf into a pinnate one. In these instances the
lobes or leaflets become separated one from another by a kind of
apostasis. This change may be frequently seen in the horse-chestnut,
particularly in the young shoots formed after the trees have been pruned
or pollarded. In the adjoining cut the intermediate stages between a
palmate or digitate leaf to a pinnate one may be seen. The specimens
from which the drawing was made were taken from the same tree at the
same time.

[Illustration: FIG. 207.--Portion of leaf of parsley, showing the change
from short wavy to long flat leaf-lobes.]

In the white clover, _Trifolium repens_, a similar transition may often
be observed, as also in some species of _Potentilla_.[514]

=Elongation of the parts of the flower.=--The only circumsta