Objectives
Key Terms While you may think of flowers as gently perfumed, not all flowers are so sweet. The so-called carrion flowers emit the stench of rotting flesh! They attract insects that normally feed on carcasses. The odor of flowers is just one adaptation that helps attract particular animal pollinators to visit, pick up, and carry away pollen to other flowers. Angiosperm Adaptations The flowers of many angiosperms are adapted in ways that attract insects or other animals that transfer pollen directly from one flower to another flower. The great variety of flower size, shape, odor, texture, and color reflects the diversity of interactions that angiosperms have with animal pollinators. Angiosperms such as grasses that are wind-pollinated also have flowers, but their flowers are typically smaller and less flashy than those pollinated by animals. The male reproductive organs of a flower are called stamens (STAY munz). At the tip of a stamen is the anther, which produces pollen grains containing the male gametophytes. The female reproductive organs of flowers are called carpels (KAHR pulz). At the base of a carpel is a chamber—the ovary. Within the ovary, female gametophytes, called embryo sacs, develop within ovules. Figure 19-16 highlights key stages in the angiosperm life cycle. (You will read about more details of this process in Chapter 20.) During reproduction, pollen lands on sticky tips of carpels, and a tube grows from each pollen grain down the carpel toward an ovule in the ovary. Two sperm cells produced within the pollen grain are then released into the female gametophyte. One sperm cell fertilizes an egg cell. This produces a zygote, which develops into an embryo. The second sperm cell fuses with nuclei in the larger center cell of the female gametophyte, which then develops into a nutrient-storing tissue called endosperm. The endosperm nourishes the embryo as it develops. This "double fertilization" that simultaneously produces a zygote and an endosperm is a characteristic of angiosperms. The whole ovule (now containing the zygote and endosperm) develops into a seed. The flowers of some species contain many ovules, and thus can produce many seeds. An angiosperm's development of seeds within ovaries contrasts with the "naked" seed development of gymnosperms. As seeds are developing from ovules, the ovary wall thickens, forming a fruit that encloses the seeds. A fruit is the ripened ovary of a flower. (In some angiosperms, other flower parts are incorporated along with the ovary into the fruit.) Fruits protect and help disperse seeds. For example, colorful fruits often attract animals that digest the fruits and deposit the seeds in their wastes, usually at some distance from the parent plant. The Diversity of Angiosperms Amborella The species Amborella trichopoda is an example of a "living fossil." It is the only surviving species of the oldest branch of the angiosperm lineage. Amborella lives on the island of New Caledonia in the South Pacific, and has been found nowhere else. Water Lilies Water lilies form another clade (evolutionary branch) of flowering plants that evolved before monocots and dicots. Star Anise and Others In addition to Amborella and water lilies, there are other early angiosperm clades that evolved before the monocots and true dicots. The star anise is a representative species. Monocots Some familiar monocots are day lilies, orchids, irises, palms, and grasses. Monocots have flower petals in multiples of 3. The monocot clade includes about 65,000 species. Dicots The true dicots include poppies, roses, peas, sunflowers, oaks, and maples. Dicots have 4 or 5 flower petals. The dicot clade includes about 165,000 species. Human Dependence on Angiosperms Early humans probably collected wild seeds and fruits for food. Agriculture developed as humans began sowing seeds and cultivating plants to have a more dependable food source. Later, humans began to breed plants to improve food quantity and quality. So far, only a tiny fraction of more than 280,000 known plant species (including non-angiosperms) have been explored for potential uses. For example, almost all of the human food supply is based on the cultivation of only about two dozen species. And, while more than 120 prescription drugs are currently extracted from plants, researchers have so far investigated fewer than 5000 plant species as potential sources of new medicines. Certain human activities are threatening plant species, sometimes before their potential uses are even known. The tropical rain forest, which is losing plant species at the fastest rate of all Earth's ecosystems, may be a medicine chest of healing plants that could become extinct before they are even discovered. Chapter 36 will explore some of the issues involved in biological conservation.
Concept Check 19.5 |