Some holly-trees bear only male flowers, which have four stamens
producing a rather small quantity of pollen, and a rudimentary pistil;
other holly-trees bear only female flowers; these have a full-sized pistil,
and four stamens with shrivelled anthers, in which not a grain of pollen
can be detected. Having found a female tree exactly sixty yards from a
male tree, I put the stigmas of twenty flowers, taken from different
branches, under the microscope, and on all, without exception, there were a
few pollen-grains, and on some a profusion. As the wind had set for
several days from the female to the male tree, the pollen could not thus
have been carried. The weather had been cold and boisterous and therefore
not favourable to bees, nevertheless every female flower which I examined
had been effectually fertilised by the bees, which had flown from tree to
tree in search of nectar. But to return to our imaginary case; as soon as
the plant had been rendered so highly attractive to insects that pollen was
regularly carried from flower to flower, another process might commence.
No naturalist doubts the advantage of what has been called the
"physiological division of labour;" hence we may believe that it would be
advantageous to a plant to produce stamens alone in one flower or on one
whole plant, and pistils alone in another flower or on another plant. In
plants under culture and placed under new conditions of life, sometimes the
male organs and sometimes the female organs become more or less impotent;
now if we suppose this to occur in ever so slight a degree under nature,
then, as pollen is already carried regularly from flower to flower, and as
a more complete separation of the sexes of our plant would be advantageous
on the principle of the division of labour, individuals with this tendency
more and more increased, would be continually favoured or selected, until
at last a complete separation of the sexes might be effected.
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