The Origin of Species by Means of Natural Selection: Or, the Preservation of Favoured Races in the Struggle for Life, 6th EditionDarwin, Charles
History
The Origin of Species by Means of Natural Selection: Or, the Preservation of Favoured Races in the Struggle for Life, 6th Edition
Darwin, Charles
Evolution (Biology); Natural selection
From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the
same manner, but entailing changes in every part of the flower. The
nectar may be stored in variously shaped receptacles, with the stamens
and pistils modified in many ways, sometimes forming trap-like
contrivances, and sometimes capable of neatly adapted movements through
irritability or elasticity. From such structures we may advance till we
come to such a case of extraordinary adaptation as that lately
described by Dr. Crüger in the Coryanthes. This orchid has part of its
labellum or lower lip hollowed out into a great bucket, into which
drops of almost pure water continually fall from two secreting horns
which stand above it; and when the bucket is half-full, the water
overflows by a spout on one side. The basal part of the labellum stands
over the bucket, and is itself hollowed out into a sort of chamber with
two lateral entrances; within this chamber there are curious fleshy
ridges. The most ingenious man, if he had not witnessed what takes
place, could never have imagined what purpose all these parts serve.
But Dr. Crüger saw crowds of large humble-bees visiting the gigantic
flowers of this orchid, not in order to suck nectar, but to gnaw off
the ridges within the chamber above the bucket; in doing this they
frequently pushed each other into the bucket, and their wings being
thus wetted they could not fly away, but were compelled to crawl out
through the passage formed by the spout or overflow. Dr. Crüger saw a
“continual procession” of bees thus crawling out of their involuntary
bath. The passage is narrow, and is roofed over by the column, so that
a bee, in forcing its way out, first rubs its back against the viscid
stigma and then against the viscid glands of the pollen-masses. The
pollen-masses are thus glued to the back of the bee which first happens
to crawl out through the passage of a lately expanded flower, and are
thus carried away. Dr. Crüger sent me a flower in spirits of wine, with
a bee which he had killed before it had quite crawled out, with a
pollen-mass still fastened to its back. When the bee, thus provided,
flies to another flower, or to the same flower a second time, and is
pushed by its comrades into the bucket and then crawls out by the
passage, the pollen-mass necessarily comes first into contact with the
viscid stigma, and adheres to it, and the flower is fertilised. Now at
last we see the full use of every part of the flower, of the
water-secreting horns of the bucket half-full of water, which prevents
the bees from flying away, and forces them to crawl out through the
spout, and rub against the properly placed viscid pollen-masses and the
viscid stigma.
Public-domain text, read in full here on John Shaqi.
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