Pollen is the dust, usually
yellow in color, produced by the stamens and made up of millions of microscopic
granules, the shape size and appearance of which are characteristic to the
individual species. Each granule is responsible for the production of male
sexual cells, the gametes. In order for the gametes to be conveyed to their
female counterparts, contained in the ovules, the pollen granule must come into
contact with the stigma. This is the process known as pollination and it
represents the most critical moment in the life of a flower. Although the
outcome of this process is strongly conditioned by chane, nevertheless
evolution has effectively reduced the accidental element by fostering certain
mechanisms that guarantee, at least statistically, the success of pollination.
The oldest and most widespread of such mechanisms involve insects which as
carriers of pollen travel from stamen to stigma and from flower to flower. The
flower reciprocates by putting food substances, namely nectar, a source of carbohydrates
and pollen itself, a source of protein at the disposal of those insects. The
plant in fact is well equipped to produce a surplus of pollen so that a part of
it can be used for fertilization.
There’s a wide range of
specialization between flowers and insects. In some flowers the nectar is
stored at the end of a long, narrow coralline tube for example, the Cape
leadwort, Plumbago auriculata, visited by insects with a very long mouth
apparatus, such as butterflies, some of these tropical flowers such as the
family Bignoniaceae are pollinated by the tiny hummingbirds. Often the
arrangement of the flower’s internal organs is such as to compel the insect to
become smeared with pollen in order to reach its food; when the insect visits
another flower are pollinated by insects and among these so called
entomophilous flowers are the zygomorphic forms. But there’re numerous species
in some cases entire families, which are Anemophilous, i.e. pollinated by the
wind. They’re characterized by many inflorescences, sometimes soft and
pendulous (Catkins or amenta) of numerous small flowers, almost or wholly
lacking a perianth and so structured and inclined as to expose their anthers
and stigmas to the slightest breath of wind. Anemophilous flowers obviously
produce and expend a much greater quantity of pollen than entomophilous flowers
because their type of pollination is that much more uncertain. The success rate
is highest in dry, windy climates. These’re also flowers such as those of
Fallopia baldschuanica of the Polygonaceae which are both entomophilous and Anemophilous,
capable of being pollinated by both methods.
Once the pollen reaches the
stigma, the sticky, sugary liquid that covers its surface attracts the pollen
granules and induces them to germinate. Within a short time each granule then
protrudes a pollen tube, which lengthens to grow through the style, digesting
and feeding on its internal tissues. Continuing its growth, the tube reaches
the ovary and eventually, the ovule. Meanwhile the embryo sac a group of a few
cells, two of which are the female gametes has formed inside the ovule. When
the pollen tube comes into contact with the embryo sac, fertilization proper
takes places. The tube releases into the sac two male gametes which unite with
their female counterparts to produce, respectively the zygote i.e. the first
embryonic cell of new seedling and the initial cell of the endosperm, the
nutritive tissue of the seed plant, the function of which is to feed the new
plant during its early stages when it is incapable of carrying gout
photosynthesis. In some seeds this food tissue does not form and in such cases initial
nutrition is accomplished by the first leaves of the seedling itself, those
already formed in its embryo, the cotyledons. 
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