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
If twice or thrice the number had been collected, the three forms would probably
have been found nearly equal; I infer this from considering the above figures,
and from my son telling me that if he had collected in another spot, he felt
sure that the mid-styled plants would have been in excess. I several times sowed
small parcels of seed, and raised all three forms; but I neglected to record the
parent-form, excepting in one instance, in which I raised from short-styled seed
twelve plants, of which only one turned out long-styled, four mid-styled, and
seven short-styled.
Two plants of each form were protected from the access of insects during two
successive years, and in the autumn they yielded very few capsules and presented
a remarkable contrast with the adjoining uncovered plants, which were densely
covered with capsules. In 1863 a protected long-styled plant produced only five
poor capsules; two mid-styled plants produced together the same number; and two
short-styled plants only a single one. These capsules contained very few seeds;
yet the plants were fully productive when artificially fertilised under the net.
In a state of nature the flowers are incessantly visited for their nectar by
hive- and other bees, various Diptera and Lepidoptera. (4/3. H. Muller gives a
list of the species ‘Die Befruchtung der Blumen’ page 196. It appears that one
bee, the Cilissa melanura, almost confines its visits to this plant.) The nectar
is secreted all round the base of the ovarium; but a passage is formed along the
upper and inner side of the flower by the lateral deflection (not represented in
the diagram) of the basal portions of the filaments; so that insects invariably
alight on the projecting stamens and pistil, and insert their proboscides along
the upper and inner margin of the corolla. We can now see why the ends of the
stamens with their anthers, and the ends of the pistils with their stigmas, are
a little upturned, so that they may be brushed by the lower hairy surfaces of
the insects’ bodies. The shortest stamens which lie enclosed within the calyx of
the long- and mid-styled forms can be touched only by the proboscis and narrow
chin of a bee; hence they have their ends more upturned, and they are graduated
in length, so as to fall into a narrow file, sure to be raked by the thin
intruding proboscis. The anthers of the longer stamens stand laterally farther
apart and are more nearly on the same level, for they have to brush against the
whole breadth of the insect’s body. In very many other flowers the pistil, or
the stamens, or both, are rectangularly bent to one side of the flower. This
bending may be permanent, as with Lythrum and many others, or may be effected,
as in Dictamnus fraxinella and others, by a temporary movement, which occurs in
the case of the stamens when the anthers dehisce, and in the case of the pistil
when the stigma is mature; but these two movements do not always take place
simultaneously in the same flower.
Public-domain text, read in full here on John Shaqi.
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