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
This species is conspicuously heterostyled, as has been noticed by several
authors. The pistil in the long-styled form is nearly twice as long as that of
the short-styled. In the latter the stigmas are smaller and, diverging to a
greater degree, pass out low down between the filaments. I could detect no
difference in the two forms in the size of the stigmatic papillae. In the long-
styled form alone the stigmatic surfaces of the mature pistils twist round, so
as to face the circumference of the flower; but to this point I shall presently
return. Differently from what occurs in L. grandiflorum, the long-styled flowers
have stamens hardly more than half the length of those in the short-styled. The
size of the pollen-grains is rather variable; after some doubt, I have come to
the conclusion that there is no uniform difference between the grains in the two
forms. The long-stamens in the short-styled form project to some height above
the corolla, and their filaments are coloured blue apparently from exposure to
the light. The anthers of the longer stamens correspond in height with the lower
part of the stigmas of the long-styled flowers; and the anthers of the shorter
stamens of the latter correspond in the same manner in height with the stigmas
of the short-styled flowers.
I raised from seed twenty-six plants, of which twelve proved to be long-styled
and fourteen short-styled. They flowered well, but were not large plants. As I
did not expect them to flower so soon, I did not transplant them, and they
unfortunately grew with their branches closely interlocked. All the plants were
covered under the same net, excepting one of each form. Of the flowers on the
long-styled plants, twelve were illegitimately fertilised with their own-form
pollen, taken in every case from a separate plant; and not one set a seed-
capsule: twelve other flowers were legitimately fertilised with pollen from
short-styled flowers; and they set nine capsules, each including on an average 7
good seeds, ten being the maximum number ever produced. Of the flowers on the
short-styled plants, twelve were illegitimately fertilised with own-form pollen,
and they yielded one capsule, including only 3 good seeds; twelve other flowers
were legitimately fertilised with pollen from long-styled flowers, and these
produced nine capsules, but one was bad; the eight good capsules contained on an
average 8 good seeds each. Judging from the number of seeds per capsule, the
fertility of the two legitimate to that of the two illegitimate unions is as 100
to 20.
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