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 a state of nature the long and short-styled forms would no doubt occur in
nearly equal numbers, as I infer from the analogy of the other heterostyled
species of Primula, and from having raised the two forms of the present species
in exactly the same number from flowers which had been LEGITIMATELY crossed. The
preponderance in Table 5.31 of the long-styled form over the short-styled (in
the proportion of 134 to 51) results from gardeners generally collecting seed
from self-fertilised flowers; and the long-styled flowers produce spontaneously
much more seed (as shown in the first chapter) than the short-styled, owing to
the anthers of the long-styled form being placed low down in the corolla, so
that, when the flowers fall off, the anthers are dragged over the stigma; and we
now also know that long-styled plants, when self-fertilised, very generally
reproduce long-styled offspring. From the consideration of this table, it
occurred to me in the year 1862, that almost all the plants of the Chinese
primrose cultivated in England would sooner or later become long-styled or
equal-styled; and now, at the close of 1876, I have had five small collections
of plants examined, and almost all consisted of long-styled, with some more or
less well-characterised equal-styled plants, but with not one short-styled.
With respect to the equal-styled plants in the table, Mr. Horwood raised from
purchased seeds four plants, which he remembered were certainly not long-styled,
but either short or equal-styled, probably the latter. These four plants were
kept separate and allowed to fertilise themselves; from their seed the seventeen
plants in the table were raised, all of which proved equal-styled. The stamens
stood low down in the corolla as in the long-styled form; and the stigmas, which
were globular and smooth, were either completely surrounded by the anthers, or
stood close above them. My son William made drawings for me, by the aid of the
camera, of the pollen of one of the above equal-styled plants; and, in
accordance with the position of the stamens, the grains resembled in their small
size those of the long-styled form. He also examined pollen from two equal-
styled plants at Southampton; and in both of them the grains differed extremely
in size in the same anthers, a large number being small and shrivelled, whilst
many were fully as large as those of the short-styled form and rather more
globular. It is probable that the large size of these grains was due, not to
their having assumed the character of the short-styled form, but to monstrosity;
for Max Wichura has observed pollen-grains of monstrous size in certain hybrids.
The vast number of the small shrivelled grains in the above two cases explains
the fact that, though equal-styled plants are generally fertile in a high
degree, yet some of them yield few seeds. I may add that my son compared, in
1875, the grains from two white-flowered plants, in both of which the pistil
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