THE LIVING WORLD
Unit Seven. Plant Life
34.2. Structure of the Flower
Flowers contain the organs for sexual reproduction in angio- sperms. Like animals, angiosperms have separate structures for producing male and female gametes (eggs and sperm), but the reproductive organs of angiosperms are different from those of animals in two ways. First, in angiosperms, both male and female structures usually, but not always, occur together in the same individual flower. Second, angiosperm reproductive structures are not permanent parts of the adult individual. Angiosperm flowers and reproductive organs develop seasonally; these flowering seasons correspond to times of the year most favorable for pollination, the transfer of pollen to the female parts of a flower.
Flowers often contain male and female parts. The male parts, called stamens, are the long filamentous structures you see in the cut-away diagram of a flower in figure 34.1a. At the tip of each filament is a swollen portion, called the anther, that contains pollen. The female part, called the carpel, is the vaseshaped structure in the figure. The carpel consists of a lower bulging portion called the ovary, a slender stalk called the style, and a sticky tip called the stigma that receives pollen. The ovary contains the egg cell. Often flowers contain both stamens and carpels, as in figure 34.1a, but there are some exceptions. Various species of flowering plants, such as willows and some mulberries, have imperfect flowers, flowers containing only male or only female parts that occur on separate plants. Plants that contain imperfect flowers with only ovules or only pollen are called dioecious, from the Greek words for “two houses.” These plants cannot self-pollinate and must rely on outcrossing. In other plants, there are separate male and female flowers, but they occur on the same plant. These plants are called monoecious, meaning “one house.” In monoecious plants, the male and female flowers may mature at different times, increasing the chances of outcrossing.
Even in plants that have functional stamens and carpels present in each flower, these organs may reach maturity at different times, which keeps the flower from pollinating itself. For example, the fireweed flowers shown in figure 34.1b, c contain both stamens and carpels, but these organs mature at different times. First, the stamens mature, as shown in figure 34.1b, and the anthers shed pollen. Then after about two days, the style elongates above the stamen, as shown in figure 34.1c, and the four lobes of the stigma are ready to receive pollen. However, all the flowers aren’t always on the same schedule. In the case of the fireweed, the lower flowers open first and so they are in their female phase when the upper flowers are opening and shedding pollen. This encourages pollen to be transferred to the stigma of another flower, promoting outcrossing.
Figure 34.1. The structure of a flower.
(a) Most flowers contain both male and female parts. The flowers of the fireweed (Epilobium angustifolium) contain both male and female structures but they mature at different times. The male flower (b) opens first. The female phase (c) follows with the elongation of the style above the stamen.
If you were to cut an anther in half, as shown above, you would see four pollen sacs, each sac containing a collection of microspore mother cells 1. Inside the pollen sacs, microspore mother cells are enclosed and protected within specialized chambers. Each microspore mother cell undergoes meiosis, but we will follow just one here that has been isolated from the chamber 2. The diploid (2n) microspore mother cell divides by meiosis, forming four haploid (1n) microspores 3. Subsequently, each microspore divides by mitosis forming pollen grains that contain a generative cell (the purple cells in the pollen grain) and a tube cell nucleus. The tube cell nucleus will form the pollen tube, and the generative cell will later divide to form two sperm cells that fertilize the female cells.
Eggs develop in the ovule of the angiosperm flower, which is contained within the ovary at the base of the carpel. Within each ovule is a megaspore mother cell 1. Each diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores 2. In most plants, only one of these megaspores, however, survives; the rest are absorbed by the ovule. The lone remaining megaspore 3 undergoes repeated mitotic divisions to produce eight haploid nuclei, which are enclosed within a structure called an embryo sac 4. Within the embryo sac, the eight nuclei are arranged in precise positions. One nucleus (the green egg cell) is located near the opening of the embryo sac. Two nuclei are located in a single cell above the egg cell and are called polar nuclei. Two nuclei reside in cells that flank the egg cell and are called synergids; and the other three nuclei are located in cells at the top of the embryo sac, opposite the egg cell, and are called antipodals.
Although all flowers contain the same basic structures, and serve the same function, they don’t all look the same. There is an amazing amount of variety in colors, shapes, sizes, and features of flowers. Figure 34.2 shows a mere sampling of the wide variety of flowers.
Figure 34.2. Different kinds of flowers.
The various flowers shown here all function in plant reproduction using the same structures, but they look very different. The birch tree flowers, in the upper left, are imperfect flowers. The male flowers are dangling down on the left and the female flowers are the clustered conelike structures on the right. The birch is a wind-pollinated plant and so the flowers are drab, lacking the brilliant colors seen in the other flowers that are insect-pollinated. These other plants use colorful flowers to attract their pollinators.
Key Learning Outcome 34.2. Sexual reproduction in angiosperms involves flowers, which contain the male and female reproductive organs. Pollen grains develop in the anther, and an embryo sac develops within the ovule.