Flowering Plants

Flowers and angiosperm success_

The angiosperms are by far the most successful plants today, with over 260,000 species and occupying most habitats on land. Most of the food plants used by humans are angiosperms - wheat, barley, apples, cabbage, lentils, peas, olives, pumpkins and many more. Angio-sperms arose during the Mesozoic, and radiated dramatically during the mid-Cretaceous.

The following are important characteristics of angiosperms:

1 The ovules are fully enclosed within carpels (Fig. 18.21c). It is believed that carpels are modified leaves that grew ovulate receptacle ovulate receptacle

integument

Figure 18.21 Evolution of the angiosperm flower: (a) cone of the Jurassic bennettitalean Williamsoniella, showing the female fertile structure, the ovule, contained in a central receptacle, and surrounded by the male fertile structures, the microsporophylls; (b) flower of the gnetale Welwitschia, showing the central ovule, and surrounding male elements; and (c) flower of the angiosperm Berberis, showing the same pattern, but with the seed enclosed in a carpel.

integument

anther ovary ovule

Figure 18.21 Evolution of the angiosperm flower: (a) cone of the Jurassic bennettitalean Williamsoniella, showing the female fertile structure, the ovule, contained in a central receptacle, and surrounded by the male fertile structures, the microsporophylls; (b) flower of the gnetale Welwitschia, showing the central ovule, and surrounding male elements; and (c) flower of the angiosperm Berberis, showing the same pattern, but with the seed enclosed in a carpel.

around the ovules, and provided a secure protective covering. In angiosperm development, the carpels grow around the ovules and fuse, although in some magnolias the carpels are not completely fused when fertilization takes place. Most angiosperm ovules have two integuments, or protective casings. Most angiosperms have pollen grains with a double outer wall separated by columns of tissue.

Angiosperms have a flower (Fig. 18.21c), a structure that is composed of whorls of sepals and petals in most. The flower includes the carpels and stamens, the male reproductive structures. The structure of flowers is not standard in all angiosperms.

5 Angiosperms all show double fertilization, that is, two sperm nuclei are involved in fertilization. One unites with the egg nucleus, while the other fuses with another nucleus that divides to form the food supply for the developing embryo. Double fertilization has been described also in Gnetales.

6 Most angiosperms have water-transporting vessels rather than just xylem tracheids. This feature, however, is absent in some magnolids and hamamelidids, and is present in gnetales.

7 Most angiosperms have a net-like pattern of veins in their leaves.

Most of these characters are regarded as typical of angiosperms, but many are not unique to angiosperms, nor are they present in all angiosperms. The only one that seems to be an acceptable apomorphy of the group is the possession of carpels around the ovule (character 1 above).

Flowers are certainly the most obvious feature of angiosperms, but several gymno-sperm groups also had organs that bear certain resemblances to flowers. Bennettitaleans (Fig. 18.21a) and Gnetales (Fig. 18.21b) have flowerlike structures with the ovule in the center, and around them structures resembling petals.

The secret of the success of the angiosperms may be the flower and the fully enclosed ovule. The carpels protect the ovule from fungal infection, desiccation and the unwelcome attentions of herbivorous insects. Double fertilization is said to offer the advantage that the parent plant does not invest energy in creating a large food store, as in gymnosperms (see Fig. 18.12), until fertilization of the ovule is assured. Pollen is produced within the anthers, which are typically borne on long filaments arranged around the centrally placed ovary or ovaries. Pollen grains are transported, by animals, often insects, or by the wind, to the stigma, and from it the pollen grains send pollen tubes to the ovules through which the sperm pass.

The petals, often brightly colored, with special fragrances and supplies of nectar (sugar water), are all adaptations of angio-sperms to ensure fertilization by insects. Some gymnosperms show hints of this pattern: the

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