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
I raised twenty plants from self-fertilised seed sent me by Mr. Scott; and they
all produced red flowers, varying slightly in tint. Of these, two were strictly
long-styled both in structure and in function; for their reproductive powers
were tested by crosses with both forms of the common cowslip. Six plants were
equal-styled; but on the same plant the pistil varied a good deal in length
during different seasons. This was likewise the case, according to Mr. Scott,
with the parent-plant. Lastly, twelve plants were in appearance short-styled;
but they varied much more in the length of their pistils than ordinary short-
styled cowslips, and they differed widely from the latter in their powers of
reproduction. Their pistils had become short-styled in structure, whilst
remaining long-styled in function. Short-styled cowslips, when insects are
excluded, are extremely barren: for instance, on one occasion six fine plants
produced only about 50 seeds (that is, less than the product of two good
capsules), and on another occasion not a single capsule. Now, when the above
twelve apparently short-styled seedlings were similarly treated, nearly all
produced a great abundance of capsules, containing numerous seeds, which
germinated remarkably well. Moreover three of these plants, which during the
first year were furnished with quite short pistils, on the following year
produced pistils of extraordinary length. The greater number, therefore, of
these short-styled plants could not be distinguished in function from the equal-
styled variety. The anthers in the six equal-styled and in the apparently twelve
short-styled plants were seated high up in the corolla, as in the true short-
styled cowslip; and the pollen-grains resembled those of the same form in their
large size, but were mingled with a few shrivelled grains. In function this
pollen was identical with that of the short-styled cowslip; for ten long-styled
flowers of the common cowslip, legitimately fertilised with pollen from a true
equal-styled variety, produced six capsules, containing on an average 34.4
seeds; whilst seven capsules on a short-styled cowslip illegitimately fertilised
with pollen from the equal-styled variety, yielded an average of only 14.5
seeds.
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