Earthworks & Systems

Evolution of Land Plants: How Did Plants Move Onto the Land?

The transition from the marine environment to the land was not easy for plants or animals. There were many problems to overcome to survive in air.

The move from water to land started with algae, and it wasn't easy.

Air does not provide buoyancy support or nutrients; it is dry compared to plant tissue, and provides little or no buffering against solar radiation or temperature changes. To move from the water to land, plants had to adapt systems that would support their weight, provide transport of water and nutrients, protection from drying out, and insulation from the sun and temperature changes. Obviously if these adaptations were not difficult, plants would have moved on to land much earlier in the geological history f the Earth. Instead, they appear rather late in the history of life on this planet.

The ancestors of modern land plants are most likely the chlorophytes or green algae. Green algae are found in marine, freshwater, and moist soil environments. These algae also have the same type of green pigment and produce the same kind of carbohydrates during photosynthesis as land plants. Some workers have suggested that green algae may have become symbiotically linked to fungi to form lichens in the early Paleozoic. Others believe that colonial green algae developed protective coverings, transport systems, roots, and reproductive systems on their own. With a poor fossil record of early land plants, it is difficult to tell exactly how the transition from water to land occurred.

University of Hawaii at Manoa

Early land plants were undoubtedly small and lived at least partly in the water for support and nutrients. With the development of better protective coverings, roots, and internal transport systems they were able to move into drier environments on land. As they grew larger they produced more secondary xylem tissue which provided more strength. This tissue is wood which helped the plants withstand the pull of gravity and the force of the wind. With the advent of woody tissue these plants could now grow to a larger size and by the Late Devonian we have the first fossil trees preserved. These trees were known as progymnosperms and grew to a height of 20 to 30 feet. They still reproduced by spores and that is why they are not true seed-bearing gymnosperms.

Fossil of the first tree, Archaeopteris, a progymnosperm. (U. California at Berkeley

The history of land plants may be divided into three major developments. The first is the transition from the ocean to the land. This is represented in the fossil record by the early spore bearing plants and early trees which formed the great coal swamps of the Middle Paleozoic. The second is the development of seed-producing, non-flowering plants such as the gymnosperms. The seed allowed these plants to move into much drier and more hostile environments in the Late Paleozoic. The third change was the development of both seed and flowers with the angiosperms, or flowering plants, in the late Mesozoic Era. Angiosperms are the dominant land plants today with the most efficient vascular and reproductive systems of all plants.  

-  Dr. Jack

 
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