The Different Forms of Flowers on Plants of the Same SpeciesDarwin, Charles
Science
The Different Forms of Flowers on Plants of the Same Species
Darwin, Charles
Fertilization of plants; Heterostylism; Plant physiology; Plants -- Variation; Pollination
1864 page 85). Herr Breitenbach found many specimens of Primula elatior growing
in a state of nature with some flowers on the same plant long-styled, others
short-styled and others equal-styled; and the long-styled form greatly
preponderated in number; there being 61 of this form to 9 of the short-styled
and 15 of the equal-styled.) I will only add that in a state of nature any
single plant of a trimorphic species no doubt produces all three forms; and this
may be accounted for either by its several flowers being separately fertilised
by both the other forms, as Hildebrand supposes; or by pollen from both the
other forms being deposited by insects on the stigma of the same flower.
EQUAL-STYLED VARIETIES.
The tendency of the dimorphic species of Primula to produce equal-styled
varieties deserves notice. Cases of this kind have been observed, as shown in
the last chapter, in no less than six species, namely, P. veris, vulgaris,
Sinensis, auricula, farinosa, and elatior. In the case of P. veris, the stamens
resemble in length, position and size of their pollen-grains the stamens of the
short-styled form; whilst the pistil closely resembles that of the long-styled,
but as it varies much in length, one proper to the short-styled form appears to
have been elongated and to have assumed at the same time the functions of a
long-styled pistil. Consequently the flowers are capable of spontaneous self-
fertilisation of a legitimate nature and yield a full complement of seed, or
even more than the number produced by ordinary flowers legitimately fertilised.
With P. Sinensis, on the other hand, the stamens resemble in all respects the
shorter ones proper to the long-styled form, whilst the pistil makes a near
approach to that of the short-styled, but as it varies in length, it would
appear as if a long-styled pistil had been reduced in length and modified in
function. The flowers in this case as in the last are capable of spontaneous
legitimate fertilisation, and are rather more productive than ordinary flowers
legitimately fertilised. With P. auricula and farinosa the stamens resemble
those of the short-styled form in length, but those of the long-styled in the
size of their pollen-grains; the pistil also resembles that of the long-styled,
so that although the stamens and pistil are of nearly equal length, and
consequently pollen is spontaneously deposited on the stigma, yet the flowers
are not legitimately fertilised and yield only a very moderate supply of seed.
We thus see, firstly, that equal-styled varieties have originated in various
ways, and, secondly, that the combination of the two forms in the same flower
differs in completeness. With P. elatior some of the flowers on the same plant
have become equal-styled, instead of all of them as in the other species.
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