plant

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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.

Yellow and the diamond shape represent Brazil's mineral wealth. The motto Ordem e Progresso means ‘Order and Progress’. Green stands for the vast forests. Effective date: 15 November 1889. >>