Virtually all other living creatures depend on plants to survive. Through photosynthesis, plants convert energy from sunlight into food stored as carbohydrates. Because animals cannot get energy directly from the sun, they must eat plants (or other animals that have had a vegetarian meal) to survive. Plants also provide the oxygen humans and animals breathe, because plants use carbon dioxide for photosynthesis and release oxygen into the atmosphere.
Plants are found on land, in oceans, and in fresh water. They have been on Earth for millions of years. Plants were on Earth before animals and currently number about 260,000 species. Three features distinguish plants from animals:
- Plants have chlorophyll, a green pigment necessary for photosynthesis;
- Their cell walls are made sturdy by a material called cellulose; and
- They are fixed in one place (they don’t move).
In order to study the billions of different organisms living on earth, biologists have sorted and classified them based on their similarities and differences. This system of classification is also called a taxonomy and usually features both English and Latin names for the different divisions.
All plants are included in one so-called kingdom (Kingdom Plantae), which is then broken down into smaller and smaller divisions based on several characteristics, including:
- Whether they can circulate fluids (like rainwater) through their bodies or need to absorb them from the moisture that surrounds them;
- How they reproduce (e.g., by spores or different kinds of seeds); and
- Their size or stature.
The majority of the 260,000 plant species are flowering herbs. To describe all plant species, the following divisions (or phyla) are most commonly used to sort them. The first grouping is made up of plants that are non-vascular; they cannot circulate rainwater through their stems and leaves but must absorb it from the environment that surrounds them. The remaining plant species are all vascular (they have a system for circulating fluids). This larger group is then split into two groups: one that reproduces from spores rather than seeds, and the other that reproduces from seeds.
Mosses and “allies,” or related species (Bryophyta and allies)
Mosses or bryophyta are non-vascular. They are an important foundation plant for the forest ecosystem and they help prevent erosion by carpeting the forest floor. All bryophyte species reproduce by spores not seeds, never have flowers, and are found growing on the ground, on rocks, and on other plants.
Originally grouped as a single division or phylum, the 24,000 bryophyte species are now grouped in three divisions: Mosses (Bryophyta), Liverworts (Hepatophyta), and Hornworts (Anthocerotophyta). Also included among the non-vascular plants is Chlorophyta, a kind of fresh-water algae.
Vascular Plants with Spores
Ferns and allies (Pteridophyta and allies)
Unlike mosses, ferns and related species have a vascular system, but like mosses, they reproduce from spores rather than seeds. The ferns are the most plentiful plant division in this group, with 12,000 species. Other divisions (the fern allies) include Club mosses or Lycopods (Lycopodiophyta) with 1,000 species, Horsetails (Equisetophyta) with 40 species, and Whisk ferns (Psilophyta) with 3 species.
Vascular Plants with Seeds
Conifers and allies (Coniferophyta and allies)
Conifers and allies (Coniferophyta and allies) Conifers reproduce from seeds, but unlike plants like blueberry bushes or flowers where the fruit or flower surrounds the seed, conifer seeds (usually cones) are “naked.” In addition to having cones, conifers are trees or shrubs that never have flowers and that have needle-like leaves. Included among conifers are about 600 species including pines, firs, spruces, cedars, junipers, and yew. The conifer allies include three small divisions with fewer than 200 species all together: Ginko (Ginkophyta) made up of a single species, the maidenhair tree; the palm-like Cycads (Cycadophyta), and herb-like plants that bear cones (Gnetophyta) such as Mormon tea.
Flowering Plants (Magnoliophyta)
The vast majority of plants (around 230,000) belong to this category, including most trees, shrubs, vines, flowers, fruits, vegetables, and legumes. Plants in this category are also called angiosperms. They differ from conifers because they grow their seeds inside an ovary, which is embedded in a flower or fruit.