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Butterflies sipping nectar from a flower.

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.

  Flower Sex Cells

Structure of the flower stamen.

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.

Structure of the flower carpel.

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.

  Flower Types

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.

Comparison of monocot and dicot flowers.

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.

  Flower Reproduction

Graphic of labeled flower parts.

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.

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