Who Are the Plants?

Plants are so different from all other kinds of organisms that they get their own kingdom, the kingdom Plantae.

There are 10 divisions, divided into two very basic categories: vascular plants and non-vascular plants (those without a special transport system to carry water and nutrients throughout the plant). The only non-vascular plants are the mosses, hornworts, and liverworts. These plants need to be in very moist surroundings, so they can never grow tall. Ferns and the 'fern allies' (mosses, hornworts, liverworts, whisk ferns, club mosses, quillworts, and horsetails) reproduce with spores (tiny single-celled structures that develop into new plants). Conifers, cycads, gingkoes, gnetophytes, and flowering plants all reproduce with seeds (larger structures that develop after a female egg is fertilized by the male sperm, delivered in pollen).

Among the seed plants, the non-flowering plants (conifers, cycads, gingkoes, and gnetophytes) are also called gymnosperms (`naked seeds') because their seeds are not generally enclosed within fruits. The flowering plants are also called angiosperms (`seeds in a vessel') because their seeds are packaged within fruits. Angiosperms are far and away the most successful contemporary plants, in part because of their ability to lure animals into transporting their pollen and seeds.

So what on earth are algae, fungi, and lichens? These organisms are sometimes studied in herbaria, like plants, but are placed in different kingdoms.

Algae, like plants, are photosynthetic and manufacture their own carbohydrates, but they are classified in the kingdom Protista. Most plants are relatively large terrestrial organisms, while most algae are microscopic aquatic organisms (although some algae, like sea lettuce, are multicellular).

Fungi are classified in the kingdom Fungi. They have long, threadlike bodies called hyphae—the familiar mushrooms that we see are really only temporary reproductive structures that produce spores. Fungi, unlike plants and algae, cannot manufacture their own food and need an external source of nutrients. They release secretions that digest their food before it is absorbed into their bodies. Fungi (along with bacteria) are the most important decomposers: although some fungi attack living plants or animals, most recycle organisms that are already dead.

Lichens are also classified in the kingdom Fungi, but they are actually mutually beneficial associations between certain fungi and photosynthetic algae or bacteria. Most of the lichen body is actually fungal, but photosynthetic cells are sandwiched inside. This collaboration enables lichens to colonize harsh, exposed habitats where few other organisms can survive.