Plant

• 1. Introduction

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  Plants make up the kingdom Plantae, and they share a single trick that powers nearly everything alive on land. Inside green pigment called chlorophyll, packed into structures named chloroplasts, they pull energy from sunlight and turn carbon dioxide and water into sugars. There are about 382,000 accepted species, ranging from single cells to the tallest trees. But what counts as a plant has never been settled. Aristotle once filed everything that was not an animal under one heading, lumping in algae and fungi. So where do the boundaries actually fall now? How did organisms that began in water come to cover almost the entire planet? And why does so much of human survival, from grain to medicine to paper, trace back to this one kingdom?

• 2. Drawing The Boundaries

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  Aristotle, who lived from 384 to 322 BC, divided living things by whether they had a "sensitive soul" or, like plants, only a "vegetative soul". His student Theophrastus carried that work forward into plant taxonomy. Much later, Linnaeus, who lived from 1707 to 1778, built the modern system of scientific classification but kept the old split, naming the plant kingdom Vegetabilia. Modern usage has narrowed the word considerably. The name Plantae now maps to one of four nested concepts, from least to most inclusive. The strictest, Plantae sensu strictissimo, means land plants alone: liverworts, hornworts, mosses, and vascular plants. A step wider, Viridiplantae or green plants, adds the green algae from which land plants emerged. These green plants share cellulose in their cell walls, chlorophylls a and b, and plastids bound by only two membranes. Wider still, Archaeplastida folds in the red algae and the glaucophyte algae, which store Floridean starch in the cytoplasm rather than the plastids. This article treats Viridiplantae as its subject, and that clade is where the next chapter begins.

• 3. From Algal Scum To Forests

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  An algal scum once coated the land, but true land plants did not appear until the Ordovician, organized roughly like today's bryophytes. Fossils of organisms with a flattened thallus in Precambrian rocks hint that multicellular freshwater eukaryotes existed over 1000 million years ago. In the late Silurian, primitive land plants began to diversify, and bryophytes, club mosses, and ferns enter the fossil record. The Rhynie chert preserves early plant anatomy in cellular detail from the early Devonian. These plants were petrified in chert that formed in silica-rich volcanic hot springs. By the end of the Devonian, roots, leaves, and secondary wood were all present in trees such as Archaeopteris. The Carboniferous brought swampy forests dominated by clubmosses and horsetails, some as large as trees, alongside the first seed plants, the early gymnosperms. The Permo-Triassic extinction event reshaped communities, perhaps clearing the way for flowering plants in the Triassic. Their later spread in the Cretaceous was so rapid that Darwin called it an "abominable mystery".

• 4. The Cell That Holds Water

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  A plant cell carries features that animal cells lack: a large water-filled central vacuole, chloroplasts, and a strong flexible cell wall sitting outside the cell membrane. Chloroplasts trace back to a symbiosis between a non-photosynthetic cell and photosynthetic cyanobacteria. The cell wall, made mostly of cellulose, lets a cell swell with water without bursting, while the vacuole lets the cell change size as its cytoplasm stays constant. Most plants are multicellular, and their cells specialize into tissues like xylem and phloem and into organs with distinct jobs. Roots absorb water and minerals, stems support and transport, leaves run photosynthesis, and flowers handle reproduction. The chemistry at the heart of it all is exact: six molecules of carbon dioxide and six of water, driven by light, yield one sugar molecule and six of oxygen. That released oxygen feeds the next story, because plants are not the only ones giving it back to the air.

• 5. Powering The Living World

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  Photosynthesis, first by cyanobacteria and later by photosynthetic eukaryotes, transformed the early Earth's anoxic atmosphere, which now holds 21% oxygen. Animals and most other organisms are aerobic and depend on that oxygen, while those that do not are confined to rare anaerobic settings. Plants stand as the primary producers in most terrestrial ecosystems and form the base of the food web there. By mass they are dominant, making up about 80% of world biomass at roughly 450 gigatonnes of carbon. Plants are also distributed almost worldwide across many biomes. Only the hardy Antarctic flora has adapted to that southern continent: algae, mosses, liverworts, lichens, and just two flowering plants. Many biomes take their names from their vegetation, including grassland, savanna, and tropical rainforest. That dominance sets up a web of partnerships and conflicts, where plants and other organisms have shaped each other over deep time.

• 6. Partners And Parasites

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  Flowering plants have evolved pollination syndromes, suites of flower traits that draw insect and bird pollinators that transfer pollen while feeding on pollen or nectar. Seeds travel by similar bargains: some fruits offer nutritious outer layers, while their seeds survive a passage through an animal's gut, and others carry hooks that catch on fur. Myrmecophytes are plants that coevolved with ants, offering a home and sometimes food, and gaining defenders against herbivores plus fertilizer from ant wastes. Most plant species link their roots to fungi in a mutualism called mycorrhiza, trading photosynthetic carbohydrates for water and mineral nutrients. Some plants instead become takers. About 1% are parasitic, ranging from semi-parasitic mistletoe, which keeps photosynthetic leaves, to fully parasitic broomrape and toothwort, which have no chlorophyll. The fungal endophyte Neotyphodium coenophialum in tall fescue grass has pest status in the American cattle industry. Hemiepiphytes like the strangler fig start as epiphytes, then root and overpower their hosts. And some 630 species, including the Venus flytrap and sundew, turn predator to digest small animals for nitrogen and phosphorus.

• 7. The Human Harvest

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  About 7,000 plant species have been used for food, yet most of today's diet comes from only 30 species. The major staples include cereals like rice and wheat, starchy roots and tubers like cassava and potato, and legumes like peas and beans. Vegetable oils such as olive oil and palm oil supply lipids, and coffee, tea, and chocolate deliver caffeine as mild stimulants. Medicinal plants are a primary source of organic compounds, and modern drugs drawn from them include aspirin, taxol, morphine, quinine, reserpine, colchicine, digitalis, and vincristine. The pharmacopoeia of Dioscorides, De materia medica, described some 600 medicinal plants. It was written between 50 and 70 CE and stayed in use in Europe and the Middle East until around 1600 CE. Plants also yield nonfood goods: essential oils, dyes, resins, rubber, latex, and the cellulose behind cotton cloth and paper. In the laboratory they became model organisms, as when Gregor Mendel bred pea plants to derive the laws of inheritance, and Barbara McClintock examined maize chromosomes to tie them to inherited traits. Ornamental plants have even bent history, as in tulipomania.