d parts, such as the stigmas, the ovules,
and the anthers, are the first to disappear, the filaments often being
developed without anthers. Ultimately a deformed and empty perianth
alone remains. In the ordinary course of things the mouth of the
perianth is open, but in some of these malformations it is closed, and
when that happens, the effects of atrophy are the more observable in the
stamens and pistils.

The impotence of the pollen in certain atrophied flowers, as noticed by
the same observer, is of much interest, especially in reference to the
sexual relationship between the different forms in polymorphic flowers
as studied by Mr. Darwin.

A change in direction may also be noted as a common accompaniment of
atrophy or suppression; thus, in a capsule of _Veronica Beccabunga_,
which was one-celled by the abortion of one carpel, the style was
lateral instead of terminal.

As to the causes of these structural deviations but little is known;
certain of them have been already alluded to. In some cases atrophy and
suppression maybe regarded as permanent states of a condition usually
transitory, but this is clearly not always the case. Among external
causes anything bringing about an enfeebled condition might be supposed
to lead to atrophy, or suppression of some parts.

Gaertner[545] attributes the arrested development and fall of flowers to
some among the following causes:--1. non-application of the pollen of
the same variety, and consequent imperfect fertilisation; 2. any
considerable injury to the calyx, &c.; 3. destruction of the style or
stigma before the fertilisation of the ovary; 4. application to the
stigma of imperfect or heterogeneous pollen or indifferent pulverulent
matter; 5. defective conceptive power in the ovary.

Abortion of the ovules is considered by the same authority to be due
to--1. deficiency of heat; 2. excess of moisture; 3. peculiar formation
of the ovary; 4. over-luxuriant development of roots or buds; 5.
peculiar conditions of cultivation; thus, cuttings and layers produce
sterile and abortive seeds much more frequently than plants of the same
species raised from seed; 6. abortion of the seed is often combined with
luxuriant development of the walls of the fruit.

Temperature and climatal changes in general seem not to be without
effect, as has been already mentioned in the case of _Arenaria
tetraquetra_, which is polygamous when growing in mountain districts.
Other illustrations of a s