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
the exact ratio, but it must have been at least as 100 to 75. A similar
trial was subsequently made with two other peas from a different cross,
and the result was nearly the same. For instance, a crossed seedling
between the Maple and Purple-podded pea was planted in poor soil and
grew to the extraordinary height of 116 inches; whereas the tallest
plant of either parent variety, namely, a Purple-podded pea, was only 70
inches in height; or as 100 to 60.
Sarothamnus scoparius.
Bees incessantly visit the flowers of the common Broom, and these are
adapted by a curious mechanism for cross-fertilisation. When a bee
alights on the wing-petals of a young flower, the keel is slightly
opened and the short stamens spring out, which rub their pollen against
the abdomen of the bee. If a rather older flower is visited for the
first time (or if the bee exerts great force on a younger flower), the
keel opens along its whole length, and the longer as well as the shorter
stamens, together with the much elongated curved pistil, spring forth
with violence. The flattened, spoon-like extremity of the pistil rests
for a time on the back of the bee, and leaves on it the load of pollen
with which it is charged. As soon as the bee flies away, the pistil
instantly curls round, so that the stigmatic surface is now upturned and
occupies a position, in which it would be rubbed against the abdomen of
another bee visiting the same flower. Thus, when the pistil first
escapes from the keel, the stigma is rubbed against the back of the bee,
dusted with pollen from the longer stamens, either of the same or
another flower; and afterwards against the lower surface of the bee
dusted with pollen from the shorter stamens, which is often shed a day
or two before that from the longer stamens. (5/16. These observations
have been quoted in an abbreviated form by the Reverend G. Henslow, in
the ‘Journal of Linnean Society Botany’ volume 9 1866 page 358. Hermann
Muller has since published a full and excellent account of the flower in
his ‘Befruchtung’ etc. page 240.) By this mechanism cross-fertilisation
is rendered almost inevitable, and we shall immediately see that pollen
from a distinct plant is more effective than that from the same flower.
I need only add that, according to H. Muller, the flowers do not secrete
nectar, and he thinks that bees insert their proboscides only in the
hope of finding nectar; but they act in this manner so frequently and
for so long a time that I cannot avoid the belief that they obtain
something palatable within the flowers.
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