The angiosperms, presently the dominant group of vascular plants, first appeared in the fossil record about 125 million ago, in the early part of the Cretaceous period. The group became dominant worldwide some 80 to 90 million ago, late in the Cretaceous period. Among the fossils formed at that time, many modern families of angiosperms and some modern genera can be recognized. The pollen of primitive angiosperms that occurred more than 125 million years ago may have been indistinguishable from gymnnosperm pollen or fern spores, so that it is difficult to be certain of the presence of angiosperms more than 125 million years ago, but the group is doubtless somewhat older.

Angiosperms may have evolved in the semiarid uplands and dry interior basins of West Gondwanaland, a supercontinent that included what are now South America and Africa. By the time these continents separated completely (about 90 millio years ago), their climates had changed considerably the angiosperms were approaching world dominance. Possible reasons for their success include various adaptations for droughs resistance, as well as the evolution of efficient often highly specialized pollination systems. (It is this same major global climatic shift that signaled the beginning of the end for the dinosaurs; perhaps, well along before the comet intervention at the Cretaceous/Tertiary boundary).

The flower is the most conspicuous feature of the angiosperms, and it has played a significant role in their evolution. The carpel is a leaflike structure that has undergone enfolding to enclose the ovules and subsequent differentiation into a basal, swollen ovary, a stalk like style, and a stigma that is receptive to pollen. Similarly, the stamens have evolved from leaflike precursors or slender, branching systems with terminal sporangia to become specialized into the slender structures that are characteristic of most living angiosperms. Sepals are specialized structures that protect the flower in bud, and petals in most angiosperms are sterilized stamens that have assumed a function in attracting insects. The petals of some angiosperms, however, were derived from sepals. The spiral arrangement of the flower parts of primitive angiosperms, coupled with possession of numerous free parts, has given way to a whorled arrangement in most modern forms, with a definite number of parts that are often fused with one another or with her whorls of the flower. Examples of specialized families are Asteraceae (the composites), in which numerous highly specialized flowers are aggregated into a head, which functions as the attractive unit for insects; and Orchidaceae (the orchids), in which bizarre elaboration of the flower parts has resulted in a highly irregular flower with the most specialized pollination systems

Pollination by insects is basic in the angiosperms, and the first pollinating agents were probably beetles or similar insects. The closing of the carpel may have been a device to protect the ovules from being eaten by visiting insects. More specialized groups of insects evolved later in the history of the angiosperms, and wasps, flies, butterflies, and moths have each left their mark on the morphology of certain angiosperm flowers. The bees, however, are the most specialized and constant of flower-visiting insects and have probably had the greatest effect on the evolution of angiosperm flowers. Each group of flower-visiting animals is associated with a particular group of floral characteristics related to the visual and olfactory senses of the animals they attract. Some angiosperms have become wind-pollinated, shedding copious quantities of small, nonsticky pollen and having well-developed, often feathery stigmas that are efficient in collecting such pollen from the air.(Most trees of the US are wind-pollinated).

Flowers that are regularly visited and pollinated by animals with high energy requirements, such as hummingbirds, must produce large amounts of nectar. They must then protect and conceal these sources of nectar from other potential visitors with lower energy requirements, which might satiate themselves with nectar from a single flower (or from the flowers of a single plant) and therefore fail to move on to another plant of the same species to effect cross-pollination. Wind pollination is inefficient, and the individual plants must grow close together in large groups for the system to work, whereas insects, birds, or bats can carry pollen great distances from flower to flower.

Fruits are as diverse as the flowers from which they are derived, and they can be classified either morphologically or anatomically or in terms of their methods of dispersal. They are basically mature ovaries, but, if additional floral parts are retained, they are said to be accessory fruits. Simple fruits are derived from one carper or a group of united carpers, aggregate fruits from the free carpers of one flower, and multiple fruits from the fused carpers of several or many flowers. Dehiscent fruits split open to release the seeds, and indehiscent ones do not.

Wind-borne fruits or seeds are light and often have wings or tufts of trichomes that aid in their dispersal. The fruits of some plants expel their seeds explosively. Some seeds or fruits are borne away by water, in which case they must be buoyant and have water-resistant coats. Others are disseminated by animals, particularly by vertebrates, and have evolved fleshy coverings that are tasty and often conspicuous to attract feeding animals. Others adhere to the coats of mammals or the feathers of birds and are distributed in this manner.

A third aspect of the evolutionary success and diversification of the angiosperms has been biochemical convolution. Certain groups of angiosperms have evolved various secondary plant substances, such as alkaloids, which protect them from most foraging herbivores. However, certain herbivores (normally those with narrow feeding habits) are able to feed on these plants and are regularly found associated with them. Potential competitors are excluded from the same plants by their toxicity. This pattern indicates that a stepwise pattern of coevolutionary interaction has occurred, and it appears likely that the early angiosperms also may have been protected by their ability to produce some chemicals that functioned as poisons for herbivores.

THE BIOLOGY OF PLANTS BY RAVEN, EVERT AND EICHHORN