life11e_ch27

Adaptations to life on land distinguish land plants from green algae

Land plants first appeared in the terrestrial environment between 450 and 500 million years ago. How did they survive in an environment that differed so dramatically from the aquatic environment of their ancestors? While the water essential for life is everywhere in the aquatic environment, water is difficult to obtain and retain in the terrestrial environment.

No longer bathed in fluid, organisms on land faced potentially lethal desiccation (drying). Large terrestrial organisms had to develop ways to transport water to body parts distant from the source of the water. And whereas water provides aquatic organisms with support against gravity, a plant living on land must either have some other support system or sprawl unsupported on the ground. A land plant must also use different mechanisms for dispersing its gametes and progeny than its aquatic relatives, which can simply release them into the water.

Survival on land was facilitated by the evolution among plants of numerous adaptations, including:

The cuticle, a membrane covered in waxes to retard water loss
Stomata, small openings in leaves and stems that open and close to regulate gas exchange and water loss
Gametangia, multicellular organs that enclose plant gametes and prevent them from drying out
Embryos, young plants contained within a protective structure
Certain pigments that afford protection against the mutagenic ultraviolet radiation that bathes the terrestrial environment
Thick spore walls containing a polymer that protects the spores from desiccation and resists decay
A mutually beneficial association with fungi (mycorrhizae) that promotes nutrient uptake from the soil

The cuticle may be the most important—and the earliest—of these features. Composed of several unique waxy lipids that coat the leaves and stems of land plants, the cuticle has several functions, the most obvious and important of which is to keep water from evaporating from the plant body.

As ancient plants colonized the land, they not only adapted to the terrestrial environment, they also modified it by contributing to the formation of soil. Acids secreted by plants helped break down rock, and the organic compounds produced by the breakdown of dead plants contributed nutrients to the soil. Such effects are repeated today wherever plants colonize and grow in new areas.