Angiosperms are seed plants and their life cycle is similar to that of pine trees (see Fig. 30.7), with a few important differences. One of these differences is that in angiosperms pollen and ovules are produced in a single structure—the flower (Fig. 30.11)— rather than in separate cones. The significance of this arrangement is that, because pollen and ovules are so close together, an animal pollinator can deliver pollen to one plant and take up pollen to carry to another in a single visit. Approximately 12% of flowering plant species produce pollen and ovules in separate flowers, but all of these species descend from ancestors whose flowers produced both. Such “unisexual” flowers are often found in the approximately 20% of angiosperm species that have reverted to wind pollination.

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Flowers are spectacularly diverse in size, color, scent, and form, but they all have the same basic organization: concentric whorls of floral organs (Fig. 30.12; Chapter 20). The outer whorls consist of sepals and petals, while the inner whorls are made up of pollen-producing stamens and ovule-producing carpels. Let’s look at the floral organs in more detail, starting with the carpels at the center of the flower.

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Carpels are modified leaves that have become folded over and sealed along the edges to form a hollow chamber. Sporangia are produced on the inner surface of this cavity (Fig. 30.13a). Within each sporangium a single cell undergoes meiosis, forming four haploid spores, one of which develops into a female gametophyte. Recall that an ovule consists of a female gametophyte surrounded by protective tissues and that ovules, when fertilized, develop into seeds. The fact that the ovules develop within the carpel is what gives rise to the name “angiosperm,” which is from Greek words meaning “vessel” and “seed.” In gymnosperms, the ovules are not enclosed, and their name, again from the Greek, literally means “naked seeds.”

Each flower produces one or more carpels (typically 3 to 5, but sometimes more than 20), but because the carpels are often fused, there may be only a single structure at the center of the flower. At the base is the ovary in which one to many ovules develop. The ovary protects the ovules from being eaten or damaged by animals, but also makes it impossible for pollen to land directly on the surface of the ovule. To reach the ovules, pollen must land on the stigma, a sticky or feathery surface at the top of the carpel(s), and grow down through the stalklike style. The female gametophytes produce chemicals that guide the pollen tubes toward unfertilized ovules. Because animal pollinators can deposit many pollen grains at once, the fastest growing pollen tubes are the most likely to deliver sperm to an egg. In some plants, the style can be more than 10 cm long—corn silks, for instance, are styles—creating the opportunity for competition, and thus natural selection, between genetically distinct male gametophytes.

Immediately surrounding the carpels are the pollen-producing stamens. A stamen can have a leaflike structure bearing sporangia on its surface, but more commonly it consists of a filament that supports a structure known as the anther, which contains several sporangia (Fig. 30.13b). Within each sporangium, many cells undergo meiosis to form haploid spores. The male gametophyte then develops within the spore wall, forming pollen. In most flowers, the anther splits open, exposing the pollen grains. Once exposed, the pollen can come into contact with the body of a visiting pollinator or, in the case of a wind-pollinated species, be carried off by the wind. However, in some plants, for example tomato, small holes open at the top of anthers. To extract the pollen, bees land on the flower and vibrate at just the right frequency to shake loose the pollen inside. Orchids and milkweeds do not release individual pollen grains. Instead, they disperse their pollen all together in a package with a sticky tag that attaches to a visiting pollinator.

The outer whorls of the flower produce neither pollen nor ovules but instead contribute to reproductive success in other ways. Most flowers have two outer whorls, of which the outermost is made up of sepals. Sepals, which are often green, encase and protect the flower during its development. In contrast, petals are frequently brightly colored and distinctively shaped. Their role is to attract and orient animal pollinators. In addition to serving as visual cues, petals in many flowers produce volatile oils. These are the source of the distinctive odors, some pleasant, some decidedly not, that many flowers use to advertise their presence to pollinators.

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Quick Check 5 What is the name and function of the structures in each whorl of a flower?