Members of the plant kingdom develop from embryos - multicellular structures enclosed in maternal tissue. Because all plants form embryos, they are all multicellular. Furthermore, because embryos are the products of the sexual fusion of cells, all plants potentially (although not always in reality) have a sexual stage in their life cycle. In the sexual stage, the male cell (sperm nucleus, haploid) fertilizes the female (egg or embryo sac nucleus, haploid). Many plants grow and reproduce in ways that bypass the two-parent sexual fusion - all must have evolved from ancestors that formed embryos by sexual cell fusion. One example of asexual reproduction is the strawberry plant; plantlets form on extensions called runners extending from the parent plant. A second example is the asexual reproduction of little green balls of cells called gemmae (Latin, "gems" or "buds") by a parent moss or liverwort plant. Evolution of the embryo, protected by maternal tissue from drying and other environmental hazards, was a major factor in the spread of plants from oceans to dry land. Development in green algal (chlorophyte) ancestors of intimate symbioses with fungi may have been another factor in transitions from aquatic to terrestrial life, facilitating uptake of minerals and water by the plant. All plants are composed of eukaryotic cells, many having green plastids. We distinguish plants from all other organisms by their life cycles rather than by their capacity for photosynthesis because some plants (beech drops, Epifagus, for example) are entirely without photosynthesis throughout their lives. Photosynthesis by plants requires enzymes within membrane-bounded plastids. All plants that photosynthesize produce oxygen. (In comparison, in photosynthetic prokaryote species, enzymes are bound as chromatophores to cell membranes, not packaged separately. Prokaryote patterns of anaerobic and aerobic photosynthesis include formation of end products such as sulfur, sulfate, and oxygen.)

Plants are adapted primarily for life on land, although many dwell in water during part of their life history. Plants are the organisms most responsible on land and in shallow marine environments for transforming solar energy, water, and carbon dioxide into photosynthate: food, fiber, coal, oil, wood, and other forms of stored energy. (In the open ocean, the plankton protoctists are the primary producers.) Houseplants, trees, and crop plants are members of the plant kingdom. Although most plants are multicellular, green, photosynthesizing organisms, a few genera such as dodder (Cuscuta) and Indian pipe (Monotropa) lost green pigment in the course of evolution and became parasitic. Many photosynthetic organisms that were once classified as members of the plant kingdom on the basis of color and sedentary habit are no longer considered plants, because they lack embryos and other minimal criteria for plant classification. Cyanobacteria (blue-green bacteria, Phylum Cyanobacteria), green algae (Phylum Chlorophyta), all other algae (for example, Phyla Chrysomonada through Phaeophyta, Rhodophyta, and Gamophyta), and lichens (Phylum Ascomycota; fungi with bacteria or protoctist symbionts) are now placed with their relatives in the bacteria, protoctista, or fungi kingdom. Photosynthesis by plants sustains the rest of the biota not only by converting solar energy into food, but also by absorbing carbon dioxide and producing oxygen.

Some half million species of plants have been described. Because new species are found each year, especially in the tropics, probably another half million plants await discovery. Furthermore, this estimate is probably low; many plants resemble each other in form and will be distinguishable as separate species only by chemical analysis.