Flowering plants

The development of the flowering plants is possibly the greatest success story in the evolution of the earth. From the time of their appearance in the middle of the Cretaceous period, about 130 million years ago, they have flourished and diversified so that today they dominate nearly all the terrestrial plant communities, from tropical forests to temperate deciduous woodlands, and from grasslands to deserts. Even the sea has flowering plants of some kind. They are also the prime producers of food for the animals of the earth (but not the sea), not least for humans who rely on them as food for themselves and domesticated livestock.

Flowering plants are grouped into the An-giospermae, or Magnoliophyta, about 250,000 species of which have been named and described. These plants are further arranged in 2 groups—the dicotyledons, or Magnoliopsida, with about 190,000 species, and the monocotyledons, or Liliopsida, with about 60,000 species. (There are, however, several systems of plant classification; the system described in this book may differ slightly from classification systems used elsewhere. For examples, see The World Book Encyclopedia articles plant (A classification of the plant kingdom] and classification, scientific.) In addition to the thousands of “natural” species, millions of forms, varieties, and cultivars have been artificially created by plant breeding programs, in order to decorate gardens and homes, and to increase the abundance and the extent of crops.

The flowering parts of plants vary, from the primitive plants in which they are complex and many, to the advanced plants in which they are simple and reduced. Sunflowers (Helian-thus annuus), like all members of the Compositae family, are higher dicotyledonous plants with only their central disk florets capable of forming seeds. The yellow, outer, petallike florets serve to attract pollinating insects and birds.

The appearance of angiosperms

The first fossils with rudimentary angiosperm characteristics, such as pollen grains, date from the Triassic period, which began about 245 million years ago. The first true angiosperms, which possibly evolved from an ancestral cycad, were most likely to have been tropical trees with flowers—possibly similar to those of a present-day magnolia—and with large, fleshy edible fruits that were attractive to birds and other animals.

These appeared in the early part of the Cretaceous period, about 130 million years ago. During this period—the climax of the Age of Dinosaurs—the land flora underwent a great change, and many of the new plants would be recognizable today. As well as the magnolias, there were willows, oaks, poplars, and sycamores. The ubiquitous grasses that make up such a familiar part of the modern landscape did not develop until Cenozoic times, when grasslands, and their accompanying running mammals, spread at the expense of the forests.

But the exact way in which the angiosperms evolved and the steps that occurred between the appearance of the first flowering plants and the production of the diversity that exists today remains a mystery. The answer lies in the huge gaps in the fossil record—possibly due to the fact that flowering plants evolved in parts of the world that were subsequently submerged—and until these gaps are filled, the precursors of the angiosperm plants will remain unknown.

Frangipani (Plumeria sp.) is a simple tropical flower. It attracts pollinating insects with its bright pink or white and yellow flowers, and also with its distinctive scent.

The characters of success

The whole design and life of the angiosperm flower, whether it is a showy bloom or an inconspicuous grass flower, is devoted to the production of viable seed from which the next generation can grow. Flowering plants differ from the other major class of plants— Gymnospermae—by having their female sex cells (ovules) enclosed in an ovary; the gymno-sperms have naked seeds. In addition, the seeds that develop from angiosperm ovules are enclosed in a fruit. But the most important feature of the angiosperm life style is that the generation of gametophytes, on which the insurance of genetic diversity depends, is reduced to the activities of a few cells only, and confined to the safe depths of the flower.

The first flowering plants were hermaphroditic—that is, they contained male pollen-producing anthers and female ovule-containing ovaries in the same flower. This arrangement is still found in most modern angio-sperms, but it seems likely that the primitive plants possessed genes that induced a state of self-sterility—the pollen from one flower was not capable of fertilizing the ovules of the same flower—which made a transfer of pollen between the flowers of different individuals of the same species essential.

Animals were originally responsible for this necessary process of the transfer of pollen from flower to flower—a role they have now played for millions of years. Only in specialized advanced states of angiosperm development has animal help been replaced by obligatory self-pollination, seed production without the need for fertilization, or by wind-pollinating mechanisms. Insects were the first pollinators to aid plants in their need—they were tempted by the pollen produced by the plants, which is rich in nutrients. In time they turned to the sugary nectar and, as the insects entered the flower in search of the nectar, they brushed against the rough-surfaced pollen, which stuck to them. On the next visit to a flower of the same species, the pollen was transferred to the receptive stigma and carried to the ovule, where fertilization was effected.

As the plants’ systems evolved, petals became brightly colored and patterned to attract birds and guide insects to the nectar. Many of these patterns are visible only in the ultraviolet wavelength, a part of the light spectrum in which insect vision can operate. Flowers also developed scents that were attractive to insects and birds. The opening of flowers and scent production became synchronized to the active periods of pollinators, and as the reliance of specific pollinators increased, so the structure of the flower became more complex. Those flowers that do not rely on animal associations for pollination do not have color, nectar, or scent because they do not need to attract animals.

Wind, animals, the explosive mechanisms of the seeds, and occasionally water, are the prime dispersers of angiosperm seed. Because the seeds have a self-contained food supply, they are also freed from the necessity of an external food source when they start to germinate; they can also remain dormant for long periods if they need to. Nevertheless, angiosperm seeds develop extremely quickly in comparison to, say, gymnosperms. A dandelion, for example, takes six weeks from seed germination to seed dispersal from a mature plant. A conifer, on the other hand, can take up to two or three years. These characteristics have made angiosperms highly adaptable and able to diversify, and account for their enormous success on this planet.

Tiger lilies (LiHum ti-grinumldiVe monocotyle-donous flowers, and, like many others, have exposed flowering parts. The six pendulous, lobed structures are the stamens (male). There are three stigmas (female) and three compartments in the ovary. The large, showy flowers are pollinated by insects that push past the protruding stamens to reach the nectar, picking up pollen as they move, and transferring it to the stigmas.