imilar character are mentioned under the head
of Heterogamy (p. 196).

Pressure has been already alluded to as one of the most obvious of the
inducing causes of atrophy and suppression.

In the case of _Ranunculus auricomus_ before cited, in which the petals
are rarely perfect, M. de Rochebrune considers that the deficiencies in
question depend, in great measure, on the amount of moisture in the
localities where the plant grows. In most places the flowers and carpels
are apt to become more or less abortive, while the leaves are luxuriant;
while, in dry places, the foliage is small, but the flowers are more
perfect. This is quite consonant with other facts relating to the
development of flowers or of leaves in general.

But while external agencies undoubtedly play some part in bringing about
these changes, it is almost certain that internal causes inherent to the
organization of the plant are more important. Mr. Darwin[546] accounts
for the existence of rudimentary organs by the operation of the general
rule of inheritance, and explains their stunted condition as the effect
of disuse, not so much, of course, in the particular flower as in its
predecessors. This disuse may be the result of the superior efficacy of
foreign pollen as contrasted with that formed in the individual flower
itself. In this way many hermaphrodite flowers tend to become
dioecious, as in _Caryophyllaceae_, _Orchidaceae_, _Plantaginaceae_,
_Primulaceae_ and other orders.

Although many of the circumstances above mentioned apply to plants whose
structure is habitually rudimentary, there is no reason why they may
not, under due restrictions, be applied to plants whose organs are only
occasionally defective.

For further remarks on the subject of Abortion, the reader is
referred to the sections relating to suppression, etc., also to
Moquin-Tandon, 'El. Terat. Veget.,' p. 120; C. Morren, "De
l'atrophie en general," in 'Bull. Acad. Belg.,' t. xviii, 1851,
part i, p. 275.

FOOTNOTES:

[522] 'El. Ter. Veg.,' p. 132.

[523] _Spinosae arbores cultura saepius deponunt spinas in hortis_, 'Linn.
Phil. Bot.,' Sec. 272.

[524] Mr. Selby, in his 'History of British Forest Trees,' p. 465, gives
the following account of the formation of this peculiar growth:--"In the
autumn the parent aphis deposits her eggs at the base of the embryo
leaves, within the bud destined to produce the shoots of the following
year. When these begin to