Infant mortality is high and life expectancy short among the flowering
plants. They not only struggle against extremes of climate, but they are
the primary food of the animal kingdom, and so pursued by creatures that
have the advantages of sight and locomotion. It is only by marvelous
fecundity and by ingenious devices for seed dispersal that plants
maintain their position on the earth.
The first objective of every plant is to produce fertile seed in as
large a quantity as the supplies of food and moisture and the length of
season will permit. Pollination, which brings about the merging of the
male and female cells, is essential to seed production. The majority of
plants combine in a single flower stamens which carry in anthers on
their tips the male element pollen and one or more pistils which hold at
their base ovaries containing the female cells. These ovaries are
reached by the pollen through the style and the stigma at its tip. The
flower may thus fertilize itself in most species, but cross-pollination
from other plants of the same species makes for more vigorous stock. The
showy petals and petal-like sepals, which draw our eyes to flowers, make
the flower conspicuous also to bees, moths, and even birds which act as
pollen bearers. Other lures to this same end are fragrances and nectar.
The detailed mechanisms by which the various plants increase the
likelihood of cross-fertilization, within the brief period that any
given set of cells is capable of fertilization, are numerous indeed and
a fascinating study.
In most plants, seed develops and becomes fully ripe in a matter of
weeks after fertilization has occurred. It is also commonplace for a
single flower to produce a seed pod or other fruit which may contain
hundreds of separate perfect seeds.
The next step is to scatter this seed over an area wide enough to reduce
the risk of all of them perishing at once, and also wide enough to keep
the survivors from competing too closely with each other for soil,
moisture and sunlight. Here again fascinating devices come into play.
Building each seed with a plume or bit of fluff at its tip so that it
can be carried far by wind, is one of the commonest tricks. Other seeds
float easily on water and so reach new sites. Other seeds invite being
eaten by birds or beasts, and depend upon a fraction of them either
being carelessly dropped before being swallowed, or having tough enough
shells to resist digestion. Quite a number of plants produce seed pods
which, when they become thoroughly dry split open with a jerk flipping
seeds over distances of several feet. Finally there are the various burs
and barbed seeds that are carried for miles by animals and by man.
Seeds thus become scattered over the earth, and so numerous and
efficient are the devices of dispersion that in the course of years the
seeds from a single plant colony, and from the successive new outlying
colonies it founds, may become spread over miles of distance. Only a few
barriers completely stop such spreading. Oceans, high mountains and
broad deserts are the most effective barriers, but even they do not
always stop every seed of every plant.
This spread of seeds pays little attention to life zone limits, or to
such interference as rivers, hills or local barren areas may present.
Over and past all of such minor obstacles the flow of seed rolls.
The final problem for the seed is how to germinate and become
established in the place it lands. If that place is totally unsuitable
for the particular species, the answer there is failure. Many seeds may
invade a locality too dry for their development. In such a case, even if
germination occurs, all such seedlings will die before a single plant
matures. Heavy frost may act as a like absolute veto to other seedlings
that venture too high in altitude or too far north in latitude for their
own limitations. By forces such as these, each species of plant stays
contained within limits beyond which it cannot become established, even
though individual seeds may in great numbers invade impossible
localities.
Mature plants may tolerate conditions which wipe out all tender
seedlings of the same species. This leads to interesting patterns of
plant distribution in semi-desert areas, such as occur in parts of
Colorado. Once or twice in a century a series of two, three, or even
five successive years may occur when the soil is moist and
extraordinarily favorable to plant growth throughout weeks or months of
the spring and summer. In these special times seeds that have invaded a
usually hostile area may, if they have retained fertility, germinate,
push their roots deep, and become so vigorous that when normal dry years
follow these particular plants live on and thrive for the remainder of
their lives, even though their own seeds fall on barren ground and the
species maintains only a precarious or temporary foothold in the area.
Governed by forces such as these, and limited by competition with each
other, plant species have for ages taken their places in the global
economy and carried out their part of the commandment to be fruitful and
multiply. Otherwise we and the animals we prey upon could not exist.