A plant is an organism belonging to the biological group Plantae, commonly understood to include mosses, ferns, conifers, flowering plants, and their relatives. Most plants are multicellular eukaryotes that obtain energy through photosynthesis and have cell walls containing cellulose. The boundaries of Plantae vary among classification systems: some restrict it to land plants, while others include related algae. Plants provide food, habitats, and many raw materials used by humans. (openstax.org)
Definition and classification
In taxonomy, “plant” does not simply mean any green or stationary organism. Fungi, once classified with plants, are now recognized as a separate kingdom. Photosynthetic bacteria also fall outside Plantae. Land plants, called embryophytes, form a lineage distinguished by the retention and nourishment of the developing embryo within parental tissues. Broader definitions include green algae or additional related algal groups. This article primarily describes land plants. (kew.org)
Major groups differ in their conducting tissues and reproductive structures. Bryophytes—mosses, liverworts, and hornworts—lack the true vascular tissues found in other land plants. Seedless vascular plants include lycophytes and ferns, including horsetails. Gymnosperms, such as conifers, produce seeds not enclosed within an ovary. Angiosperms, or flowering plants, possess flowers and produce seeds enclosed within fruits. These categories describe branching relationships, not a ladder from “lower” to “higher” organisms. (openstax.org)
Cells, tissues, and growth
A typical living plant cell contains a nucleus, mitochondria, and other membrane-bound structures. Its wall, rich in cellulose, supplies mechanical support. A large central vacuole stores substances and helps maintain the internal pressure that keeps nonwoody tissues firm. Photosynthetic cells contain chloroplasts, although many cells, including most root cells, do not. Chloroplasts originated through an ancient association with photosynthetic bacteria, explained by endosymbiotic theory. (openstax.org)
Most vascular plants have roots, stems, and leaves. Roots anchor the organism and absorb water and mineral nutrients; stems support aboveground organs; leaves commonly provide the main photosynthetic surface. These organs can be modified for storage, climbing, or reproduction. Growth occurs through regions of actively dividing cells called meristems. Apical meristems lengthen roots and shoots, while lateral meristems produce additional thickness in many woody plants. (openstax.org)
Two tissues provide long-distance transport. Xylem carries water and dissolved minerals and contributes structural support. Phloem distributes sugars and other organic compounds from sources, such as mature leaves, to growing or storage tissues. Water loss from leaves through transpiration helps draw water upward through xylem. Adjustable pores called stomata regulate gas exchange and water loss. (openstax.org)
Energy and environmental responses
Photosynthesis uses light energy to build organic compounds from carbon dioxide and water, releasing oxygen. Chlorophyll and other pigments absorb light within chloroplasts. Plants also perform cellular respiration, breaking down organic compounds to supply usable energy for cellular processes; this continues in both daylight and darkness. Some parasitic plants obtain organic nutrients from other organisms and have reduced or lost photosynthetic capacity. (openstax.org)
Plants detect and respond to light, gravity, temperature, and touch. Plant hormones, including auxins, gibberellins, ethylene, and abscisic acid, coordinate development and responses to environmental conditions. Directional growth responses include shoots bending toward light and roots growing in response to gravity. Chemical defenses and physical structures can reduce damage from herbivores and pathogens. (openstax.org)
Reproduction and life cycles
Land plants exhibit alternation of generations: their life cycle contains multicellular haploid and diploid phases. The diploid sporophyte produces haploid spores through meiosis. Spores develop into gametophytes, which produce eggs or sperm through mitotic division. Fertilization joins gametes, restoring the diploid condition and initiating a new sporophyte. The gametophyte is conspicuous in bryophytes, whereas the sporophyte dominates in vascular plants. (openstax.org)
Seeds protect embryos and usually contain food reserves. Pollen allows sperm delivery without the external film of water required by many seedless plants. In flowering plants, pollination transfers pollen to a receptive stigma. Subsequent double fertilization produces both an embryo and nutritive endosperm. Fruits protect seeds and frequently assist their dispersal by animals, wind, or water. (openstax.org)
Many plants also reproduce asexually through runners, rhizomes, bulbs, tubers, or fragments. Such offspring are generally genetically similar to the parent, apart from mutations. Plant life spans range from annuals, which complete their life cycle within one growing season, to perennials that survive and reproduce over multiple years. (openstax.org)
Evolution, ecology, and human uses
Land plants evolved from green algal ancestors. Their evolution involved adaptations to drying, support against gravity, gas exchange, and reproduction outside water. Fossils documenting vascular tissues date to about 430 million years ago. Seeds, pollen, flowers, and fruits subsequently enabled further diversification across terrestrial environments. (openstax.org)
Plants support food webs as primary producers within many ecosystems. They participate in the carbon cycle by incorporating carbon into biomass, and in the water cycle through water uptake and transpiration. Associations with root-inhabiting fungi help many species obtain nutrients and water. Plants also provide humans with cereals, fruits, timber, fibers, fuels, and chemical compounds. (openstax.org)
Habitat loss, invasive species, and climate change threaten plant biodiversity. Conservation includes protecting habitats, documenting species and distributions, maintaining living collections, and storing suitable seeds in seed banks. These approaches preserve both individual species and genetic variation relevant to future research and crop breeding. (kew.org)