The Effects of Cross & Self-Fertilisation in the Vegetable KingdomDarwin, Charles
Science
The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom
Darwin, Charles
Fertilization of plants; Plants -- Variation; Pollination
Ten flowers crossed with pollen from plants known to be distinct
seedlings, yielded nine capsules, containing on an average 34.2 seeds,
with a maximum of seventy-seven in one. Ten flowers self-fertilised
yielded eight capsules, containing on an average only 13.1 seeds, with a
maximum of twenty-five in one. This gives a ratio of 100 to 38 for the
average number of seeds per capsule for the crossed and self-fertilised
flowers. The flowers hang downwards, and as the stigmas stand close
beneath the anthers, it might have been expected that pollen would have
fallen on them, and that they would have been spontaneously
self-fertilised; but these covered-up plants did not produce a single
capsule. On some other occasions uncovered plants in the same greenhouse
produced plenty of capsules, and I suppose that the flowers had been
visited by bees, which could hardly fail to carry pollen from plant to
plant.
The seeds obtained in the manner just described were placed on sand, and
after germinating were planted in pairs,--three crossed and three
self-fertilised plants on the opposite sides of four pots. When the
leaves were 2 or 3 inches in length, including the foot-stalks, the
seedlings on both sides were equal. In the course of a month or two the
crossed plants began to show a slight superiority over the
self-fertilised, which steadily increased; and the crossed flowered in
all four pots some weeks before, and much more profusely than the
self-fertilised. The two tallest flower-stems on the crossed plants in
each pot were now measured, and the average height of the eight stems
was 9.49 inches. After a considerable interval of time the
self-fertilised plants flowered, and several of their flower-stems (but
I forgot to record how many) were roughly measured, and their average
height was a little under 7.5 inches; so that the flower-stems on the
crossed plants to those on the self-fertilised were at least as 100 to
79. The reason why I did not make more careful measurements of the
self-fertilised plants was, that they looked such poor specimens that I
determined to there them re-potted in larger pots and in the following
year to measure them carefully; but we shall see that this was partly
frustrated by so few flower-stems being then produced.
These plants were left uncovered in the greenhouse; and the twelve
crossed plants produced forty capsules, whilst the twelve
self-fertilised plants produced only five; or as 100 to 12. But this
difference does not give a just idea of the relative fertility of the
two lots. I counted the seeds in one of the finest capsules on the
crossed plants, and it contained seventy-three; whilst the finest of the
five capsules produced by the self-fertilised plants contained only
thirty-five good seeds. In the other four capsules most of the seeds
were barely half as large as those in the crossed capsules.
TABLE 6/91. Cyclamen persicum: 0 implies that no flower-stem was
produced.
Heights of plants measured in inches.
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