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plant

Organism that carries out photosynthesis, has cellulose cell walls and complex cells, and is immobile. A few parasitic plants have lost the ability to photosynthesize but are still considered to be plants.

Plants are autotrophs, that is, they make carbohydrates from water and carbon dioxide, and are the primary producers in all food chains, so that all animal life is dependent on them. They play a vital part in the carbon cycle, removing carbon dioxide from the atmosphere and generating oxygen. The study of plants is known as botany.

Levels of complexity

The simplest plants consist of a body, or thallus, on which the organs of reproduction are borne. Simplest of all are the threadlike algae, for example Spirogyra, which consist of a chain of cells.

The seaweeds (algae) and mosses and liverworts (bryophytes) represent a further development, with simple, multicellular bodies that have specially modified areas in which the reproductive organs are carried. Higher in the morphological scale are the ferns, club mosses, and horsetails (pteridophytes). Ferns produce leaflike fronds bearing sporangia on their under surface in which the spores are carried. The spores are freed and germinate to produce small independent bodies carrying the sexual organs; thus the fern, like other pteridophytes and some seaweeds, has two quite separate generations in its life cycle (see alternation of generations).

The pteridophytes have special supportive water-conducting tissues, which identify them as vascular plants, a group which includes all seed plants, that is the gymnosperms (conifers, yews, cycads, and ginkgos) and the angiosperms (flowering plants).

Seed plants

The seed plants are the largest group, and structurally the most complex. They are usually divided into three parts: root, stem, and leaves. Stems can grow above or below ground. Their cellular structure is designed to carry water and salts from the roots to the leaves in the xylem, and sugars from the leaves to the roots in the phloem. The leaves manufacture the food of the plant by means of photosynthesis, which occurs in the chloroplasts they contain. Flowers and cones are modified leaves arranged in groups, enclosing the reproductive organs from which the fruits and seeds result.

Conservation status

The World Conservation Union published a world list of threatened plants in 1998. The list includes 33,798 species of vascular plants (flowering plants, ferns, cycads, and conifers); in other words, one in eight of the estimated 270,000 species of vascular plants are now endangered.

In 1998 there were 3,214 species of rare and endangered plants in the USA.

Plant fossils The earliest fossil evidence of land plants comes from spores, resembling the spores of some lower plants. US researchers announced, in March 2000, the oldest known fossils of land plants discovered to date. The plant spores are between 500 and 510 million years old and were found in the Grand Canyon, Arizona.

Cooksonia pertoni, a tiny fossil plant only a few centimetres high and 400 million years old, is considered to be the ancestor of all higher (vascular) plants. Fossil evidence found in Shropshire, UK, demonstrates many features critical to higher plants, most notably small strengthened tubes which allowed the plants to support themselves, grow upright, and transport water to the cells at the top of their branches.

The first plant to have its genome sequenced was thale cress Arabidopsis thaliana, completed in 2000. It has 26,000 genes, only 3,000 of which have been studied in detail.

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