Ecological rewards and pressures may have enticed ancient relatives of green algae to move onto land. Benefits of this migration would include better exposure to sunlight, and easier access to carbon dioxide. These are both needed for efficient photosynthesis. Pitfalls of this move would include the need for extensive structural support, protective outer coverings, and more advanced reproductive strategies. In addition, shorter duration and protection of haploid forms may have been pressured to counteract the detrimental effects of ultraviolet radiation.
The fossil record and living examples indicate these plants overcame the challenges of moving onto land in a stepwise fashion. The sequence of this venture involved the following groups:
- Bryophytes
- Ferns
- Gymnosperms
- Angiosperms
Bryophytes Take the Leap from Water
Bryophytes are the earliest terrestrial plants. Features setting them apart from other groups are lack of both vasculature and lignins for structure. This limits their size and potential of complexity. Instead of producing seeds, they rely on flagellated sperm and eggs (like green algae), creating alternate haploid and diploid generations. Flagellated sperm require moisture so these plants are limited to damp environments. Mosses and liverworts are examples of modern bryophytes.
Ferns are the First Examples of Vascular Plants
Ferns represent a monumental step up from the lowly bryophytes. While still relying on flagellated sperm and alternation of generations for reproduction, they are vascular and incorporate lignins in their structure to raise up from the ground. They also have defined roots, stems, and leaves with specialized functions. Since they enlist flagellated sperm they still require moist environments. Millions of years ago when the earth enjoyed a warmer and wetter climate, ferns flourished with an abundance of species. The fossilized remains of these plants now contribute to coal and other fuels we utilize for energy. Since ferns require warm, moist environs, they are now mostly found in tropical or temperate rainforests.
Gymnosperms Make Seeds and Pollen
Gymnosperms are vascular plants representing the first evolutionary step in the creation of seeds. Gymnosperm (naked seed) refers to seeds that are not enclosed in special chambers. Cone-bearing plants (Conifers) reside in this group and have additional adaptations for terrestrial life which include pollen creation and reduction of the gametophyte period. Conifers create male and female cones that house the gametes. Unlike the flagellated sperm of earlier plants, gymnosperm produce immobile pollen instead of motile sperm. This pollen is created in the smaller male cones, then dispersed by wind. When pollen connects with the eggs in female cones fertilization occurs, and seeds are formed within the cone. The seeds are then released in large quantities. This strategy is much more versatile than using flagellated gametes since moisture is not required and the pollen may travel great distances using the earth's atmosphere. Thus, conifers are found throughout many regions of the world. Representatives of the gymnosperms include the many species of pine trees.
Flowering Plants
Angiosperms (contained seeds) refined the adaptations mentioned above but have evolved a further specialized element to promote the dispersal of pollen, and subsequent fertilization. This element is of course, the flower, and dispersal occurs by attracting and using animals like insects, birds, and mammals. There are at least 250,000 flowering plant species and they represent the most evolved adaptation for living in terrestrial environments.
Resources
Murray Nabors, Introduction to Botany (Benjamin Cummings, 2003).
Douglas E. Soltis and Pamela S. Soltis and Douglas E. Soltis, “The origin and diversification of angiosperms,” American Journal of Botany 91 (2004): 1614–1626.
Join the Conversation