Anthophyta / angiosperms: flowering plants

The angiosperms, or flowering plants, also known as the anthophytes, are by far the most successful group of living plants, totalling some 250,000 described species. It is their flowers and fruits that differentiate them from all other plants. The earliest known angiosperm-like plants first appear in the fossil record during the Jurassic, about 150 million years ago. But there is some speculation that angiosperms may have existed as long ago as the Permian, about 250 million years ago. It was not until the Cretaceous, 100 million years ago, that there was a rapid decline in the dominance of the gymnosperms. Then, suddenly, and from that time on, the fossil record reflects the diverse spectrum of angiosperms dominating most flora throughout the world.

Among all angiosperms, the diploid sporophyte, retaining and nourishing the gametophyte, dominates the life cycle. And it is the small gametophyte that retains and nourishes the immature eggs and non-motile sperm during development. Flowering plants are heterosporous, having two different sized spores, and oogamous, having sperm and eggs. Their zygotes develop into seeds and fruit that are highly evolved for protection and dispersal. One of their most distinguishing characteristics is the unique reproductive system involving flowers. The floral structures coevolved with the plant’s pollinating vectors, which are modes of transferring pollen from one flower to another, such as the wind, insects, and other animals. These vectors appear to have helped angiosperm dispersal into habitats where other plants might not have reached so readily.

Early angiosperms were probably pollinated by the wind. The ovule of modern gymnosperms exudes a sticky substance that traps wind-borne pollen grains. Similarly, the first angiosperms probably had such a pollinating mechanism.

Some insects, such as certain beetles, may have become dependent on readily accessible sticky, sugary droplets, produced by ovules. Insects traveling from one ovule to another may have inadvertently carried pollen with them, conferring a reproductive advantage to some plants, helping to pass on the genes of plants that had larger nectar-secreting organs (nectaries) and other structures that lured insects. Plants with certain scents and brightly colored flower parts, arranged in ways that signalled potential pollinators, had selective advantages. These are all insects that specialize in nectar consumption. Because certain insects carried pollen from flower to flower, many became vital to their host plants. Both the plants and the insects benefited from this relationship, and they coevolved. By the early Cenozoic, about 65 million years ago, many modern groups of flowers, as well as the bees, wasps, moths, and butterflies that pollinated them, had already evolved.

Comprehension check

1. Choose the right variant for the multiple-choice statements.

1. The conifers include

1. Pines b. spruce c. fir

d. all of the above e. none of the above.

2. The conifers include

1. Cypress b. redwood c. larch

d. all of the above e. none of the above.

3. The conifers include

1. Hemlock b. junipers c. yews

d. all of the above e. none of the above.

4. The … are, in terms of numbers of species, the most successful group of plants.

1. Angiosperms b. gymnosperms c. ginkgoes

d. club-mosses e. ferns.

5. The earliest known angiosperm-like plants first appeared … .

1. about 100 million years ago b. about 250 million years ago

c. about 150 million years ago d. two centuries ago

e. 100 years B.C.

6. Angiosperms are pollinated by

1. the wind b. bees c. wasps

d. moth e. all of the above.