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KINGDOM: PLANTAE (PLANTS)

Skip the introduction and jump to the plant links:
general information | taxonomy | images | bryophytes (liverworts, hornworts, mosses) |
ferns and fern allies | gymnosperms (conifers, cycads, ginko) | flowering plants | trees

Plants are the very foundation of life on earth. Along with cyanobacteria and algae, plants convert light energy from the sun into chemical energy stored in carbohydrates like starches and sugars. They accomplish this through a process called photosynthesis, which requires chemical compounds called photosynthetic pigments. By far the most common of these pigments is chlorophyll, which gives plants their green color.

All other organinisms depend on the photosynthesizers for survival. Unable to obtain energy directly from the sun, animals must eat plants (or one another) and break down the energy contained in their tissues. The final link in the cycle are decomposers like bacteria and fungi who convert dead plants and animals back into raw materials which new plants can use again. Thus the whole web of life is driven by the sun's light, trapped and stored by the plants. (The one exception are some bacteria which can obtain energy directly from chemicals in their environment.)

Besides harnessing solar energy, plants perform a second vital function: they produce oxygen. The process of photosynthesis uses carbon dioxide and releases oxygen. This is the opposite of what happens when we breathe- we take in oxygen and release carbon dioxide. Except for a few anaerobic bacteria and yeasts, all organisms "breathe" on a cellular level. Thus all organisms need oxygen. Plants take up some of the excess carbon dioxide in the air, and replenish the oxygen as it is used up, thereby helping to maintain the composition of the atmosphere.

Most scientists agree that early in the earth's history, there was no oxygen in the atmosphere. Millions of years of photsynthetic activity by plants and their ancestors probably helped to oxygenate the air, creating the conditions that allowed other life forms (like people!) to evolve. (Newer evidence suggests that geologic events were also very important in oxygenating the atmosphere. See Headline Universe.) For a timeline of early (pre-Cambrian) earth history, see Geologic History.

Plants have numerous, often complex interactions with other organisms. Herbivores, pollenators, nitrogen-fixing bacteria, and micorrhizal fungi all have intimate relationships with plants. Many insects, birds, and mammals have evolved closely with the particular plant or plants which they pollenate or eat. For some examples of plant interactions with other organisms, try these links: Nitrogen Fixing Plants?, Mycorrhizae: Plant-Fungus Partners, Pollenation, Shade and Forest Insects, Arizona Hummingbird Portfolio, and Great Plains Buffalo Association.

General Plant Information

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Plant Taxonomy

(See also Flowering Plant Taxonomy.)

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Plant Images

(See also Flowering Plant Images.)

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Bryophytes

The bryophytes are distinguished by having a gametophyte which constitutes the larger, more conspicuous generation. In most plants, the sporophyte is the dominant form, and the gametophyte generation is reduced to merely an ovum and a pollen grain/tube. (For a diagram of a plant life cycle, see alternation of generations.) Bryophytes are non-vasular plants, meaning they lack xylem and phloem, which are found in higher plants. They also lack true roots, anchoring themselves instead with filamentous rhizoids. Bryophytes do not produce seeds or flowers.

Three Divisions of plants are considered bryophytes. Division Hepatophyta consists of about 6,000 species known as liverworts. These are the simplest plants on earth. Division Anthocerophyta contains only about 100 species called hornworts which are probably close relatives of the green algae. Division Bryophyta includes about 9,500 species of mosses.

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Ferns and Fern Allies

The four Divisions in this group are seedless vascular plants. They have xylem and phloem, but do not produce seeds or flowers. Division Psilotophyta has only two genera whose members are known as whisk ferns. One species of this group is distinguished as being the only plant with neither roots nor leaves! Division Lycophyta or Lycopodiophyta, contains about 1,000 species of club mosses and tree scales. These plants are chacterized by special leaves called microphylls. During the Carboniferous period 360 to 286 million years ago, the lycophytes and the ferns were the dominant vegetation in coal-forming forests. Division Sphehnophyta or Equisetophyta has only one genus with 15 species known as horsetails. These may be the oldest species of plants on earth! The largest and most diverse group of plants outside of the flowering plants are the ferns, Division Pterophyta, with 11,000 species. Most ferns occur in the tropics.

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ALL SPECIES | BACTERIA | PROTISTS | FUNGI | PLANTS | ANIMALS
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