The Sex Life Of Flowers
Excerpted from: Managing Alternative Pollinators
Plants, like animals, have a sex life.
But unlike animals, plants can’t walk, swim, fly, or crawl
around in search of a mate. For procreation, most plants need assistance, so they make use of
sexual devices.
As with animals, the individual male and female sex cells contained within flowers are called
gametes. The male gamete is contained within small granular storage structures
called pollen.
Pollen is produced at the end of a slender stalk, or filament, on a structure called
an anther. Together anther and filament form the male component of the flower,
or stamen.
The female component, or pistil, consists of three parts: an ovary, usually at the
base of the flower, from which arises a stalk called the style, bearing a sticky
landing pad called a stigma to which pollen adheres.
The ovary of a flower contains one or more unfertilized ovules. Upon fertilization an ovule joins
with sperm to form a single cell called the zygote. The entire structure of the flower
is adapted with this process as the goal. As this zygote grows, it divides into a multi-celled
structure that we recognize as a seed.
In some plants, ovary walls, called the pericarp, enlarge to form a fruit containing
the seeds. These fruits may be soft and fleshy like an apple, or dry and hard like a walnut.
Whatever its form, without fertilization, fruit will not develop. In fruits that contain multiple seeds,
all ovules must be fertilized for the fruit to reach its maximum size.
In addition to the structures listed above, a flower may have other parts. Some of these include
a series of short green leaves, called sepals, which form a protective calyx
around an unopened flower bud. The petals, or corolla, together with the sepals,
form the flower’s perianth. The entire flowering structure is borne on the receptacle
of the pedicel, or flower stalk.
There are tremendous variations in flowers between plants of different species, and various
ways of categorizing plants based on flower characteristics. Among the most basic classification
is separation based on the number of flower parts.
Monocotyledons (monocots), including grasses, orchids, lilies, and palms, consist of flower
parts (such as stamens or petals) in groups of three.
Dicotyledons (dicots), such as most broad leaf plants, contain flower parts in groups of
four or five.
Flowers containing only the bare minimum number of parts (as opposed to some multiple of the
basic number), are called Simple Flowers.
A second way to categorize flowers is based on the presence or absence of typical flower structures.
Tulips for example lack the protective green sepals found on other flowers, and instead have modified
green leaves that slowly change color to resemble petals. These tepals are in fact neither flower nor
petal, and hence tulips are described as an incomplete, as opposed to a complete, flower
Flowers that contain both male and female reproductive parts, both stamen and pistil, are sometimes
called perfect flowers. Flowers bearing only the male or female parts are called
imperfect.
In addition, some plants will bear both separate male and female flowers (such as squash), or flowers
with both male and female parts (such as apple) on a single plant. These are called monoecious.
Other plants (including willow, holly, and sumac) have female and male flowers on separate plants.
These are called dioecious, and plants with both flower types are needed for pollination
to occur
Finally, flowers may occur individually or as clusters of multiple flowers called an inflorescence.
Sometimes the flowers of an inflorescence will be fused into a single structure that is sometimes mistaken
for a single flower. Sunflowers, coneflowers and dandelions are classic examples of this, called a composite
flower, are made up of hundreds of individual flowers.
| Monocot Versus Dicot Flowers |
| Characteristics |
Monocot Flowers |
Dicot Flowers |
| Number of Flower Parts |
Flower parts occur in threes or multiples of threes. |
Flowers have flower parts that occur in fours and fives or their multiples. |
| Petals |
The number of petals in monocot flowers usually is either three or six. In some cases, the petals might be fused. |
The number of petals in dicot flowers is four or five or their multiples. |
| Pollen Grains |
Pollen grains of monocot flowers have a single pore or furrow. |
Pollen grains of dicot flowers have three pores or furrows. |
| Perianth |
Some monocot plants might have a perianth (undifferentiated calyx and corolla). |
Dicot plants have differentiated calyx and corolla. |
| Pollination |
Most of the monocot flowers are usually wind-pollinated. |
Most of the dicot flowers are usually insect-pollinated. |
Reproduction really begins when a pollen grain comes to rest on a flower’s stigma. If the pollen grain is
compatible with the host flower, then pollen germination begins. In the case of many plants, the gametes
are self-incompatible, meaning that the pollen must come from another plant.
Each individual grain of pollen consists of two cells. After transfer, one of these cells elongates,
forming a long microscopic tube that penetrates down through the length of the style, growing closer to
the ovules located at the base of the pistil. Elongation of this pollen tube is rapid, and is fueled by nutrients
and hormones supplied by the pistil.
Eventually the pollen tube enters a tiny pore in the ovule which terminates at an unfertilized egg. While one
cell of the pollen grain forms this pollen tube, the second cell divides to form two sperm cells. The first of
these sperm cells travels down the pollen tube, completing fertilization of the waiting egg, and forming the
zygote. The second sperm cell also travels down the pollen tube and combines with another cell in the ovule
to form a food storage tissue called endosperm. Together the zygote and endosperm form the
embryo.
As the embryo matures, it remains enclosed in the ovary, which slowly enlarges to form the completed seed.
When pollen is not transferred to the receptive stigma, no seed is formed. Many flowers, such as apples, may
contain multiple ovules. If only a little pollen is transferred between flowers, not all of the eggs may become
fertilized. The result is fewer seeds with correspondingly smaller fruit. These problems can occur when
pollination vectors, such as bees, are absent.
Take this quick quiz and how much you know about the process of pollination. This quiz is intended for
fun, in a random-facts-can-be-cool kind of way. Give it a try!
Flower pollination is one of the crucial events in the life cycle of many flowering plants. When a
pollen grain moves from the anther of a flower to the stigma, pollination happens.
How does flower follination work? TIt all begins in the flower. o be pollinated, pollen must be
moved from a stamen to the stigma.