The role of flowers in plant reproductionRepresenting
innocence and beauty, flowers have been bestowed with symbolic meaning and revered for
their mystical powers in every culture. And no wonder—close your eyes for a moment
and picture the ethereal blue of a morning glory, the sensuous curves of a rosebud, the
alluring scent of the gardenia. What mystery and magic they hold for us!
But, alas, this is a course in botany, not poetry. These
wonderful colors, forms, and fragrances, botanically speaking anyway, are simply highly
specialized lures, evolved over millennia to attract the insects or other creatures on
which the plant relies for pollination. The allure they hold for us is only incidental.
A flower is a specialized shoot, bearing special
"leaves" (petals), designed solely for reproduction. Of course this technical
definition gives no hint about their beauty. Let’s dig a little deeper, and perhaps
we’ll find some poetry in the science of flowers—in the endless
diversity, intricate architecture, incredible adaptations, and fantastic maze of
interdependence among species.
The Basics. Before we can talk about the
role of flowers in reproduction, we need to cover some of the fundamentals.
There are two ways plants reproduce: sexually and
asexually. The distinction between these two types of reproduction is very important
because it affects a species’ ability to adapt to changing environments. Keep the
following two points in mind as you read on.
A good example of a plant that uses both sexual and
asexual strategies to reproduce is the common violet (Viola spp.) Violet plants
produce a number of relatively large, showy flowers; these flowers attract insects that
transfer pollen as they travel from plant to plant in search of nectar. Successful
pollination results in the fusion of gametes, so this is a type of sexual reproduction,
and the offspring are genetically unique.
However, violets have evolved a backup plan in the event
that insects aren’t around to do the pollinating. The plants also produce smaller
flowers set close to the ground. These flowers are self-pollinating—in fact, they
never even open. Still, because we have pollination—and the union of gametes, this is
also a type of sexual reproduction. In this case, the offspring are still unique, but are
most likely genetically very similar to the parent.
And, finally, to virtually guarantee the creation of new
plants, violets also produce creeping stems. These eventually sprout along their length,
forming new plants that are genetically identical to the parent. This is a form of asexual
propagation. No wonder violets are so prolific—and sometimes even invasive!
Now let’s start to explore in more detail what we
mean by terms like "genetically identical" and "genetically unique" by
looking at what happens to plant cells during reproduction. This week we are going to
focus on sexual reproduction—the method of reproduction that involves the fusion of
egg and sperm. Keep in mind that we are talking specifically about flowering
plants—rather than the more primitive mosses, for example. Though some of the
processes are the same for both groups, it will save us from having to add qualifiers such
as "except for this or that group of plants".
