A series of changes in the reproductive biology among some heterosporous plants during the late Devonian overcame this environmental restriction and allowed them to colonize a much wider range of habitats. These changes also led to the evolution of seed plants. In seed plants, the megaspore is retained in the megasporangium and the microspore is taken to a pollen chamber at the tip of this organ. The megasporangium is then sealed, and gametophyte development and fertilization occur within a protected environment. When such a megasporangium is enclosed in a seed coat, the structure with its enclosed embryo is called a seed. Seeds of the earliest such plants were exposed to the external environment; those “naked seed” plants are referred to as gymnosperms.
From the late Devonian through the base of thelate Cretaceous Period (about 382.7 million to 66 million years ago), gymnosperms underwent dramatic evolutionary radiations and became the dominant group of vascular plants in most habitats. Extant gymnosperms include conifers, cycads, and Ginkgo biloba, but those represent only a small fraction of the gymnosperms that inhabited Earth during the Mesozoic Era (251.9 million to 66 million years ago). Among the Mesozoic forms were species with a wide variety of mechanisms for effecting pollination, protecting the seeds, dispersing the seeds, and increasing the natural selection of the most successful varieties.
Primitive forms of the flowering plants (angiosperms) arose from among this diverse array of complex gymnosperms. From their earliest diversification in the Cretaceous Period (145 million to 66 million years ago), angiosperms rapidly came to dominate the land flora. Today there are more than 300,000 species of flowering plants, which account for more than 90 percent of the diversity of vascular plants. Among the many structures that contribute to this success are flowers, fruits, complex vein patterns in the leaves, and highly specialized cells of the conducting system.
Just as an ecological succession of plant forms can transform bare ground into a complex plant community, so an evolutionary succession can be thought of as having transformed bare continents into rich terrestrial biotas. In ecological succession, herbaceous plants colonize bare earth and modify the environment for the development of more complete ground cover. This further modifies the environment for the successive establishment of larger herbs, perennial shrubs, fast-growing trees, and, finally, slower-growing trees, vines, and epiphytes (plants that grow on other plants rather than in soil).
As discussed above, primitive land plants of the late Silurian and early Devonian periods were primarily small herbs that inhabited the moist lowlands near oceans, lakes, and streams. The first of these grew on bare ground because there would have been little or no organic soil, because plants had yet to produce the organic matter that gave rise to organic soils. They may be thought of as the evolutionary equivalent of primary colonizing “weeds,” and their establishment on land was responsible for the first of three dramatic increases in the diversity of land plants.
After the initial production of organic soils by these evolutionary colonizers, communities of larger herbs and shrubs were able to develop, and they became common in the middle Devonian. In the late Devonian, these communities were themselves succeeded by communities dominated by heterosporous tree-sized plants. During the early Carboniferous Period, nonseed plants continued to dominate many wetland habitats, whereas communities dominated by gymnosperm trees colonized drier habitats than had been previously available to the heterosporous forms.
Communities dominated by trees were the first to provide suitable habitats for vines and for epiphytes. Such communities also provided habitats for herbivorous animals. As a result, biological communities of considerable complexity had evolved by the late Carboniferous Period (about 323.2 million to 298.9 million years ago), giving rise to a second dramatic increase in the diversity of land plants.
With the rapid diversification of the gymnosperms and an increasing complexity of interactions between plants and animals, there was a significant evolutionary turnover among land plants during the Permian, Triassic, and Jurassic periods (298.9 million to 145 million years ago). New groups of gymnosperms replaced some of the primitive gymnosperms as well as the most prominent nonseed plants from earlier periods. Among the most successful gymnosperms of those periods were conifers, cycads, Ginkgo, and several major groups with no extant representatives.
Flowering plants probably also originated during this time, but they did not become a significant part of the fossil flora until the middle of the Cretaceous Period. The fossil evidence provides a clear picture of the rapid diversification and spectacular rise to dominance of the angiosperms during the late Cretaceous Period. This was accompanied by a dramatic increase in the complexity of plant community structure, produced at least in part by an expanding array of interactions between plants and animals. Improved pollination and seed dispersal were among the benefits of such interactions to plants.
That animals equally benefited is evidenced by the coevolution of several groups of animals (particularly insects) and angiosperms during the late Cretaceous and Paleogene periods (about 100.5 million to 23 million years ago). Such interactions contributed significantly to a third rapid increase in global plant diversity and helped angiosperms achieve the overwhelming dominance of the land flora characteristic of modern vegetation.