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
In the long-styled form of L. grandiflorum, the almost parallel or slightly
diverging anthers and stigmas project a little above the tube of the somewhat
concave flower; and they stand directly over the open space leading to the drops
of nectar. Consequently when insects visit the flowers of either form (for the
stamens in this species occupy the same position in both forms), they will get
their foreheads or proboscides well dusted with the coherent pollen. As soon as
they visit the flowers of the long-styled form they will necessarily leave
pollen on the proper surface of the elongated stigmas; and when they visit the
short-styled flowers, they will leave pollen on the upturned stigmatic surfaces.
Thus the stigmas of both forms will receive indifferently the pollen of both
forms; but we know that the pollen alone of the opposite form causes
fertilisation.
(Figure 3.5. Long-styled form of L. perenne var. Austriacum in its early
condition before the stigmas have rotated. The petals and calyx have been
removed on the near side. (3/3. I neglected to get drawings made from fresh
flowers of the two forms. But Mr. Fitch has made the above sketch of a long-
styled flower from dried specimens and from published engravings. His well-known
skill ensures accuracy in the proportional size of the parts.)
In the case of L. perenne, affairs are arranged more perfectly; for the stamens
in the two forms stand at different heights, so that pollen from the anthers of
the longer stamens will adhere to one part of an insect’s body, and will
afterwards be brushed off by the rough stigmas of the longer pistils; whilst
pollen from the anthers of the shorter stamens will adhere to a different part
of the insect’s body, and will afterwards be brushed off by the stigmas of the
shorter pistils; and this is what is required for the legitimate fertilisation
of both forms. The corolla of L. perenne is more expanded than that of L.
grandiflorum, and the stigmas of the long-styled form do not diverge greatly
from one another; nor do the stamens of either form. Hence insects, especially
rather small ones, will not insert their proboscides between the stigmas of the
long-styled form, nor between the anthers of either form (Figure 3.5), but will
strike against them, at nearly right angles, with the backs of their head or
thorax. Now, in the long-styled flowers, if each stigma did not rotate on its
axis, insects in visiting them would strike their heads against the backs of the
stigmas; as it is, they strike against that surface which is covered with
papillae, with their heads already charged with pollen from the stamens of
corresponding height borne by the flowers of the other form, and legitimate
fertilisation is thus ensured.
Thus we can understand the meaning of the torsion of the styles in the long-
styled flowers alone, as well as their divergence in the short-styled flowers.
Public-domain text, read in full here on John Shaqi.
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