Life Science: Session 4
The Transition From Water to Land
How did land plants evolve?
There is overwhelming evidence that the ancestors of modern land plants evolved in aquatic environments, where they existed and diversified over millions of years. From one group of these organisms — probably ancestors of modern species of protists known as green algae — emerged a new branch on the tree of life. From this branch arose four groups of land plants, including the mosses, the ferns, the conifers, and the flowering plants. These groups are believed to represent a sequence that reflects the evolutionary history of land plants.
What adaptations occurred with the transition from water to land?
As plants evolved from aquatic to terrestrial environments, several obstacles stood in the way. One obstacle was structural support. In water, organisms are buoyant and the effects of gravity are minimal. Even among larger forms, like kelps, structures with gas-filled vesicles allow them to float. On land, however, if a plant is to grow tall, it needs to withstand the forces of gravity. What adaptations allowed plants to get tall? Rigid cell walls developed to provide support, as did different types of supportive tissues — the woody tissue layers of trees are good examples. The mosses lack these tissues, and are thus limited to a “lowly” existence on the ground. This type of support is found in the ferns, but is fairly primitive. There are thus few ferns that grow more than a meter above the ground. It is in the conifers and flowering plants that we observe the most well-developed adaptations of this nature.
Transport of materials
A second challenge to life on land was the distribution of water and other materials to each cell. In aquatic forms, transport occurs directly from the surrounding environment. On land, however, plants must get water and other materials from the soil. What adaptations allowed taller plants to obtain these materials? During plant evolution, systems of “conducting vessels”—tube-like structures—developed that function to transport materials up, down, and around the plant. A differentiation of plant parts also evolved: leaf cells that specialize in making food; stem and branch cells to provide support; and root cells to transport nutrients from the soil. This differentiation into “true” leaves, stems, and roots made it possible for different parts of plants to fulfill different roles — all of which provide support for plants to live away from water and become tall at the same time.
A third challenge during the transition to land involved bringing sex cells together. In water, sperm are able to swim directly to eggs. On land, this can only happen in moist environments—and this is exactly what happens with mosses and ferns. Other land plants, however, exist away from such environments. What adaptations allow fertilization to occur in these plants? The answer is found in alternation of generations (described above). The sporophyte generation—the adult generation that produces spores— produces microscopic gametophytes within special structures that provide water and nutrients. The male gametophytes, which form non-swimming sperm, develop within pollen grains. The female gametophytes, which produce eggs, develop on scales (in conifers) or within ovaries (in flowering plants). Pollen is adapted to use wind to transport sperm to eggs, which replaces the need for water.
Development and dispersal of the embryo
A fourth challenge for land plants resulted in the evolution of seeds. In aquatic environments, a fertilized egg can develop into an embryo that is never in danger of dehydrating. In addition, the embryo can receive water and nutrients directly from the surrounding environment. The opposite is true on land. On land, an embryo can dry out rapidly and exists in an environment where water and nutrients exist in the ground. Seeds represent adaptations that conquer these challenges. Seeds enclose an embryo in a moist environment. And, tissues within seeds provide food for a developing embryo. Finally, seeds represent a way of dispersing the young of plants away from water as well as away from the parent plant. The seed plants include the conifers and flowering plants. Mosses and ferns, which do not produce seeds, still depend on water for the above functions.
Transition From Water to Land
|Stiff support tissue||absent||present||advanced||advanced|
|True leaves, stems, and roots||absent||present||advanced||advanced|
|Seeds||absent||absent||exposed on scale||enclosed in fruit|
|Flowers and fruits||absent||absent||absent||present|
|Water required for fertilization||yes||yes||no||no|
|prev: alternation of generations||next: flowers and pollination|