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Biology 103
Web Reports 1997
From Serendip
When one thinks of flowers, one thinks of their form, the beauty of a flower’s bright colors and symmetrical shape, rather than its function. Yet biologically speaking it is the function of the flower which determines its form. As arbitrary, absurd, and unique nature may at times seem, form follows function in nature. Flowers are not merely beautiful decorations, they are the reproductive organs of plants (1). Seeds and fruits do not exist to simply provide foods for animals. Seeds are plant embryos and the fruits in which they are contained are plant ovaries (2). The shapes of flowers, seeds, and fruits, as well as the different structures they posses, are directly related to the reproductive functions they are meant to carry out. A flower possesses four main structures, or organs, the sepal, the petals, the carpel, and the stamens. These structures can in turn be broken down into smaller structures with more specific functions. The first main organ, the sepals, are the outermost organ of the flower. They resemble leaves, and can be found at the base of the flower bud in a mature flower. During the process of maturation, the sepals surround the flower and provide protection during this time when the flower is the most vulnerable to the environment (3). The petals of a flower, collectively known as the corola, are the flower organs which receive the most attention, because of the beauty they possess. Petals contain pigments other than chlorophyll, which is the only pigment found in most plants and causes them to appear green to our eyes. The bright, attractive colors of flower petals are necessary for reproduction to occur in many plants. Petals act as advertisements for the insects and animals who aid some flowers in the process of sexual reproduction (3).
The sepals and petals surround the actual reproductive organs of the flower. As with humans, there are different male and female reproductive organs in flowers, but unlike humans, it is common to find both the male and the female reproductive organs in one flower. The ovary is located at the center of the flower and is connected to a stalk called the style, which ends in a sticky tip called the stigma. Together, these three structures make up the female reproductive organ of a flower, known as the carpel. The female reproductive cells, or ovules, are formed in the ovary (1). Surrounding the carpel in a ring are the stamens, the male reproductive organs of a flower (4). The stamens look like little stalks with enlarged organs at the end called anthers, where the male cells are produced (1). Pollen, the yellow dust which reeks havoc during allergy season, is actually made up of millions of male reproductive cells (4).
Sexual reproduction occurs in plants when the pollen produced on the anther of the stamen is received on the stigma of another flower of the same species. The pollen then travels down the style to the ovary where it fertilizes the ovum which the ovary has produced. This process produces a seed with the genetic information from two different plants, a “mother” plant and a “father” plant. The seed has the capability to become a new plant under the proper environmental conditions, and continue the cycle of reproduction (4).
Plants can also reproduce asexually and there are even some plants which are incestuous, meaning the pollen of one flower can fertilize the ovum of the same flower. However, these methods create clones of the parent plants, and fail to contribute to the diversity which nature encourages (1). Most plants have developed certain mechanisms which ensure that pollen cannot fertilize the ovum of the same flower, a process known as self-pollination. First of all, on most flowers the stamen and the stigma are located far enough apart to make self-pollination very difficult to occur (4). In some plants, the carpels and stamens of the same flower will mature at different times so that the stigma will not be receptive when the pollen is released, or the pollen will still be maturing when the stigma is receptive. Another mechanism plants use to prevent self-pollination involves chemical recognition. The stigma must chemically recognize the pollen which arrives as belonging to the same species before the process goes any further, and in some plants, the stigma also tests to make sure that the pollen did not come from the same flower. Some plants have even evolved in more dramatic ways to prevent self-pollination. Willows, marijuana, dates, and papayas all have flowers which each possess the reproductive organs for only one sex. None of the flowers of these plants possess both carpels and stamens (3).
Plants which employ one or more of these mechanisms to prevent self-pollination, must find a way to get the pollen from one flower to the stigma of another flower, a process known as cross-pollination. There are two main ways in which flowers achieve cross-pollination. Some plants use the help of certain animals or insects, known as pollinators, to transport the pollen of one flower to another flower. Some examples of pollinators, include bees, which are the most commonly used pollinator, as well as butterflies, wasps, flies, bats, as well as some birds, such as the hummingbird (1). These pollinators are lured to flowers by the scent, color, and the presence of a desirable substance, such as nectar. Flowers which use animal pollination are often large, showy, and fragrant and produce a complex form of pollen (5). To get the nectar, which is located at the center of the flower, pollinators are forced to go between the stamens and end up covered in pollen once they have collected the nectar. As the pollinator then goes on to collect more nectar from another flower, the pollen from the first flower will hopefully land on the stigma, resulting in fertilization (4).
Another method used to achieve cross-pollination is wind-pollination. Flowers which use wind pollination are usually small, simple and do not produce nectar (5). These species of flowers often lack sepals and petals as well, for they get in the way of the pollen which is floating through the wind and could prevent this pollen from reaching a flower stigma. Enormous amounts of pollen are produced by the flowers which use this method to ensure that some of the pollen produced reaches another flower after it has been released and carried off by the wind (3).
Websites
1) Flower Sex FAQ: Introduction to Pollination
3) Plant Reproduction and Development, from North Seattle Community College
4)) Why Do Plants Have Flowers?", for kids by the California Native Plant Society
5) Botany 201: Taxonomy of Flowering Plants, from Department of Biology, Texas A and M
6) Flower Dissection Lab, from University of California, San Diego
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