The phylum Angiospermae consists of the flowering plants, also commonly known as the angiosperms. This highly diverse phylum includes more than 257,000 species. The oldest evidence of angiosperms dates back to the early Cretaceous period, about 140 million years ago. The angiosperms radiated explosively and, over a period of only about 60 mil
lion years, became the dominant plant life of the planet. In later chapters, when we mention "plants," we are generally referring to the angiosperms.
The female gametophyte of the angiosperms, consisting of just seven cells, is even more reduced than that of the gym-nosperms. Thus, the angiosperms represent the current extreme of an evolutionary trend that runs throughout the tra-cheophytes: The sporophyte generation becomes larger and more independent of the gametophyte, while the gameto-
phyte generation becomes smaller and more dependent on the sporophyte.
A number of synapomorphies (shared derived traits) characterize the angiosperms:
► They have double fertilization.
► They produce a triploid nutritive tissue called the endosperm.
► Their ovules and seeds are enclosed in a carpel.
► Their xylem contains vessel elements and fibers.
► Their phloem contains companion cells.
Double fertilization was long considered the single most reliable distinguishing characteristic of the angiosperms. Two male gametes, contained within a single microgametophyte (pollen grain), participate in fertilization events within the megagametophyte of an angiosperm. One sperm combines with the egg to produce a diploid zygote, the first cell of the sporophyte generation. In most angiosperms, the other sperm nucleus combines with two other haploid nuclei of the female gametophyte to form a triploid (3n) nucleus. This nucleus, in turn, divides to form a triploid tissue, the endosperm, that nourishes the embryonic sporophyte during its early development.
Double fertilization occurs in nearly all present-day angiosperms. We are not sure when and how it evolved because there is no known fossil evidence on this point. It may have first resulted in two embryos, as it does in the three existing genera of Gnetophyta: Ephedra, Gnetum, and Wel-witschia. Both of the fertilizations in gnetophytes produce diploid products.
The name angiosperm ("enclosed seed") is drawn from another distinctive character of these plants: The ovules and seeds are enclosed in a modified leaf called a carpel. Besides protecting the ovules and seeds, the carpel often interacts with incoming pollen to prevent self-pollination, thus favor-
The pistil, containing one or more carpels, receives pollen.
Stigma Style Ovary Ovule
Stigma Style Ovary Ovule
Receptacle ing cross-pollination and increasing genetic diversity. Of course, the most evident diagnostic feature of angiosperms is that they have flowers. Production of a fruit is another of their unique characteristics.
Most angiosperms are also distinguished by the possession of specialized water-transporting cells called vessel elements in their xylem, but these cells are also found, in anatomically different form, in gnetophytes and a few ferns. A second distinctive cell type in angiosperm xylem is the fiber, which plays an important role in supporting the plant body. Angio-sperm phloem possesses another unique cell type, called a companion cell. Like the gymnosperms, woody angiosperms show secondary growth, producing secondary xylem and secondary phloem and growing in diameter.
In the following sections we'll examine the structure and function of flowers, evolutionary trends in flower structure, the functions of pollen and fruits, the angiosperm life cycle, the two major groups of angiosperms, and the origin and evolution of flowering plants.
If you examine any familiar flower, you will notice that the outer parts look somewhat like leaves. In fact, all the parts of a flower are modified leaves.
A generalized flower (for which there is no exact counterpart in nature) is diagrammed in Figure 30.7 for the purpose of identifying its parts. The structures bearing microsporan-gia are called stamens. Each stamen is composed of a filament bearing an anther that contains pollen-producing mi-crosporangia. The structures bearing megasporangia are the carpels. A structure composed of one carpel or two or more fused carpels is called a pistil. The swollen base of the pistil, containing one or more ovules (each containing a megaspo-rangium surrounded by its protective integument), is called the ovary. The apical stalk of the pistil is the style, and the terminal surface that receives pollen grains is the stigma.
In addition, a flower often has several specialized sterile (non-spore-bearing) leaves. The inner ones are called petals (collectively, the corolla) and the outer ones sepals (collectively, the calyx). The corolla and calyx, which can be quite showy, often play roles in attracting animal pollinators to the flower. The calyx more commonly protects the immature flower in bud. From base to apex, the sepals, petals, sta-
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