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 the pistil is about twice as long as that of the short-
styled, and the stamens differ in a corresponding, but reversed, manner. The
stigma is considerably more elongated and rougher than that of the short-styled,
which is smooth and almost spherical, being somewhat depressed on the summit;
but the stigma varies much in all its characters, the result, probably, of
cultivation. The pollen-grains of the short-styled form, according to
Hildebrand, are 7 divisions of the micrometer in length and 5 in breadth;
whereas those of the long-styled are only 4 in length and 3 in breadth. (1/10.
After the appearance of my paper this author published some excellent
observations on the present species ‘Botanische Zeitung’ January 1, 1864, and he
shows that I erred greatly about the size of the pollen-grains in the two forms.
I suppose that by mistake I measured twice over pollen-grains from the same
form.) The grains, therefore, of the short-styled are to those of the long-
styled in length as 100 to 57. Hildebrand also remarked, as I had done in the
case of P. veris, that the smaller grains from the long-styled are much more
transparent than the larger ones from the short-styled form. We shall hereafter
see that this cultivated plant varies much in its dimorphic condition and is
often equal-styled. Some individuals may be said to be sub-heterostyled; thus in
two white-flowered plants the pistil projected above the stamens, but in one of
them it was longer and had a more elongated and rougher stigma, than in the
other; and the pollen-grains from the latter were to those from the plant with a
more elongated pistil only as 100 to 88 in diameter, instead of as 100 to 57.
The corolla of the long-styled and short-styled form differs in shape, in the
same manner as in P. veris. The long-styled plants tend to flower before the
short-styled. When both forms were legitimately fertilised, the capsules from
the short-styled plants contained, on an average, more seeds than those from the
long-styled, in the ratio of 12.2 to 9.3 by weight, that is, as 100 to 78. In
Table 1.10 we have the results of two sets of experiments tried at different
periods.
TABLE 1.10. Primula Sinensis.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of good Capsules produced.
Column 4: Average Weight of Seeds per Capsule.
...
Column 5: Average Number of Seeds per Capsule as ascertained on a subsequent
occasion.
Long-styled by pollen of short-styled. Legitimate union :
24 : 16 : 0.58 :: 50.
Long-styled by own-form pollen. Illegitimate union :
20 : 13 : 0.45 :: 35.
Short-styled by pollen of long-styled. Legitimate union:
8 : 8 : 0.76 :: 64.
Short-styled by own-form pollen. Illegitimate union :
7 : 4 : 0.23 :: 25.
SUMMARY:
The two legitimate unions together :
32 : 24 : 0.64 :: 57.
The two illegitimate unions together :
27 : 17 : 0.40 :: 30.
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