12.8–12.10

Flowering plants are the most diverse and successful plants.

The bleeding heart flower (Lamprocapnos spectabilis) is a popular garden plant.

The appearance of flowering plants (angiosperms) about 135 million years ago in the Cretaceous period set the stage for the botanical world we know today, with flowering trees, flowering bushes, and all the grasses and herbaceous (non-woody) plants we see around us. The vast majority of plants on earth are flowering plants in the angiosperm group (FIGURE 12-19). Many of the early flowering plants would look familiar to us, and angiosperms dominate the plant world now, with some 250,000 species, compared with approximately 1,000 species of gymnosperms.

Figure 12-19Snapshot of the angiosperms.

Flowers come in a bewildering variety of sizes, shapes, and colors, but they all have similar structures: a supporting stem with modified leaves—the flashy petals and the sepals, which form a (usually) green wrapping that encloses the flower while it is in bud. Most angiosperms combine the male and female reproductive structures in the same flower. The male structure is the stamen and includes the anther, which produces the pollen, and its supporting stalk, the filament. The female reproductive structure is the carpel. It has an enclosed ovary at its base, which contains one or more ovules in which eggs develop; a stalk (the style), extending from the ovary; and a sticky tip (the stigma) (FIGURE 12-20).

Figure 12-20A flower houses a plant’s reproductive structures.

Pollination, in angiosperms, is the transfer of pollen from the male reproductive structures to the female reproductive structures of a flower—sometimes the same flower, and sometimes a different flower (on the same plant or on a different plant). Pollination is a multi-step process in which a pollen grain sticks to the stigma, forms a tube that grows until it reaches the ovule, and thus provides a route for sperm to travel down the tube to fertilize the egg. In the next section, we explore the tremendously varied methods by which angiosperms get the male gametes to the female gametes.

Although most flowers contain both male and female structures, several thousand species of plants produce flowers that are only male or only female. Sometimes, male and female flowers are borne on different individual plants, but in quite a few angiosperm species, the same plant has some male flowers and some female flowers, often side by side. Maize (corn) is a familiar plant with male and female flowers in different places on the same plant, although you might not recognize the reproductive structures as flowers. The tassel at the top of the plant is the male flower, and the ear is formed by female flowers.