Plant Adaptations to Life on Land
Organisms in water do not face many of the challenges that terrestrial creatures do.
Water supports the organism, the moist surface of the creature is a superb surface for
gas exchange, etc. For organisms to exist on land, a variety of challenges must be met.
1. Drying out. Once removed from water and exposed to air, organisms must deal
with the need to conserve water. A number of approaches have developed, such
as the development of waterproof skin (in animals), living in very moist
environments (amphibians, bryophytes), and production of a waterproof surface
(the cuticle in plants, cork layers and bark in woody trees).
2. Gas exchange. Organisms that live in water are often able to exchange carbon
dioxide and oxygen gases through their surfaces. These exchange surfaces are
moist, thin layers across which diffusion can occur. Organismal response to the
challenge of drying out tends to make these surfaces thicker, waterproof, and to
retard gas exchange. Consequently, another method of gas exchange must be
modified or developed. Many fish already had gills and swim bladders, so when
some of them began moving between ponds, the swim bladder (a gas retention
structure helping buoyancy in the fish) began to act as a gas exchange surface,
ultimately evolving into the terrestrial lung. Many arthropods had gills or other
internal respiratory surfaces that were modified to facilitate gas exchange on
land. Plants are thought to share common ancestry with algae. The plant solution
to gas exchange is a new structure, the guard cells that flank openings (stomata)
in the above ground parts of the plant. By opening these guard cells the plant is
able to allow gas exchange by diffusion through the open stomata.
3. Support. Organisms living in water are supported by the dense liquid they live in.
Once on land, the organisms had to deal with the less dense air, which could not
support their weight. Adaptations to this include animal skeletons and specialized
plant cells/tissues that support the plant.
4. Conduction. Single celled organisms only have to move materials in, out, and
within their cells. A multicellular creature must do this at each cell in the body,
plus move material in, out, and within the organism. Adaptations to this include
the circulatory systems of animals, and the specialized conducting tissues xylem
and phloem in plants. Some multicellular algae and bryophytes also have
specialized conducting cells.
5. Reproduction. Organisms in water can release their gametes into the water,
where the gametes will swim by flagella until they encounter each other and
fertilization happens. On land, such a scenario is not possible. Land animals
have had to develop specialized reproductive systems involving fertilization when
they return to water (amphibians), or internal fertilization and an amniotic egg
(reptiles, birds, and mammals). Insects developed similar mechanisms. Plants
have also had to deal with this, either by living in moist environments like the
ferns and bryophytes do, or by developing specialized delivery systems like
pollen tubes to get the sperm cells to the egg.
(Source: http://www.cartage.org)
Questions:
1) List the 5 main adaptations to life on land.
2) Why and how did the lung evolve?
3) How do aquatic organisms typically reproduce?
4) Why did xylem and phloem develop?
5) What are stomata and what are their function?
6) How do plants prevent themselves from drying out?
7) Why are skeletons necessary for most terrestrial animals?