Plants

This page covers what plants are, how flowering plants make seeds, how plants turn light into food, and how satellites watch vegetation from orbit. It draws on two US agencies: the National Park Service (NPS) and NASA, whose Earth-observing satellites measure plant growth worldwide.

What a plant is

Plants are eukaryotes, the same broad group that includes fungi, protists and animals.^[1] What sets them apart is a rigid wall around every cell and the ability to make their own food, capturing light and turning it into sugar, starch and other foods.^[1] They are one of the most abundant and varied groups of living things on Earth.^[1]

Stems, leaves and veins

The parts of a plant include roots, stems, leaves, flowers and fruits.^[2] The stem is the main supporting structure: it grows from the roots underground and holds up the rest of the plant.^[3] A herbaceous stem is soft and green, while a woody stem is hard and covered in bark.^[3] A simple leaf has one continuous blade, while a compound leaf is divided into small leaflets.^[3]

Vascular plants have systems of veins that carry water and nutrient fluids through the plant.^[4] Trees, shrubs, grasses, flowering plants and ferns are all vascular.^[4] Nonvascular plants such as mosses absorb water through membranes instead of roots, although a few mosses have similar vascular structures.^[4]

Flowering plants, flowers and seeds

Flowering plants, or angiosperms, make up more than 90% of all land-plant species, by an NPS estimate.^[1] Flowers and enclosed seeds are their key reproductive features.^[1]

A flower has female and male parts.^[5] The female part sits in the centre and is made up of the stigma, the style and the ovary.^[5] The male parts are the stamens, each made of a stalk called a filament and an anther that carries the pollen.^[5] Green, leaf-like sepals protect the flower bud, and the coloured petals together form the corolla.^[5]

Pollination is the transfer of pollen from an anther to a flower; the sticky stigma captures the arriving pollen.^[5] Pollen sticks to the sticky stigma, germinates there and grows down the inside of the style towards the ovary.^[5] After fertilization, the ovules inside the ovary develop into seeds, and the ovary becomes the fruit.^[5]

Brightly coloured flowers draw pollinating insects to the plant.^[4] Some plants depend on animals to pollinate them or to move their seeds.^[2] A seed has a protective coating and a ready supply of nourishment.^[1] Flowering-plant seeds travel on the wind, down rivers and streams, in burrs caught on animal fur (or socks), or inside fleshy fruit that animals eat and later pass out.^[1]

Photosynthesis: food from light

Plants capture and store the Sun’s energy through photosynthesis.^[6] Using that energy, they combine carbon dioxide from the air with water to make sugar and oxygen.^{[8, 7]} Chlorophyll, the substance that colours plants green, is what makes this possible.^[7] In simplified form, the reaction is CO₂ + H₂O + energy → CH₂O + O₂, where CH₂O stands for sugar.^[8]

Plants then break the sugar down again to get the energy they need to grow, which returns carbon dioxide and water: CH₂O + O₂ → CO₂ + H₂O + energy.^[8] While making their food, plants also release the oxygen we breathe.^[6] Their growth also depends on enough carbon dioxide, fresh water, nutrients and warm temperatures.^[6]

Plants and the carbon cycle

Carbon flows between rocks, the ocean, the air, plants and soil in the carbon cycle, which has slow and fast parts.^[8] Plants and phytoplankton are the main parts of the fast cycle, which moves somewhere between 10¹⁵ and 10¹⁷ grams of carbon a year.^[8]

Forests store carbon in their wood, stems and leaves, so clearing them releases it.^[8] As of 2008, deforestation accounted for about 12 percent of all human carbon dioxide emissions.^[8] By around 2010, carbon dioxide in the air had risen from about 280 parts per million before the Industrial Revolution to 387, a 39 percent increase.^[8] In a figure from around 2011, land plants had taken up approximately 25 percent of the carbon dioxide people had added to the air.^[8]

With more carbon dioxide available, plants can grow more, an effect called carbon fertilization.^[8] Models suggest growth might rise by 12 to 76 percent if carbon dioxide doubled, provided nothing else, such as water shortage, holds it back.^[8] Scientists do not know how large the effect is in the real world, because plants need more than carbon dioxide to grow.^[8]

Watching plants from space

Chlorophyll strongly absorbs visible light, while the cell structure of leaves strongly reflects near-infrared light, so comparing the two shows how green the ground is.^{[9, 11]} One measure is the Normalized Difference Vegetation Index (NDVI), developed in 1979 by a NASA researcher.^[10] It is written NDVI = (NIR − VIS) / (NIR + VIS), where NIR and VIS are the near-infrared and visible light reflected.^[9] The result always lies between −1 and +1: zero means no vegetation, and 0.8 to 0.9 means the densest possible green leaves.^[9]

Leaf area index (LAI) is the leaf area per unit of ground, seen from above; a canopy where two leaves block the view of each spot of ground has an LAI of two.^[10]

The AVHRR instrument has pixels about 1 square kilometre across, while MODIS, on the Terra satellite launched in December 1999, reaches up to 250 m resolution with almost daily global coverage.^[9] Vegetation maps help track deforestation, desertification and drought.^[9]

How plants came onto land

Most experts agree that the ancestors of land plants evolved first in the sea, then in fresh water, and finally on land.^[1] Rocks from the Ordovician Period hold evidence that plants began colonizing dry land then, but the first unquestioned plant fossils come from the Late Silurian.^[1] Flowering plants appear in the fossil record more than 100 million years ago, in the Cretaceous Period, having evolved from a specialized group of seed ferns.^[1] They quickly became the dominant plant life on land, and they remain so today.^[1]

Common misconceptions

“Plants only give out oxygen.” Plants take in carbon during photosynthesis but also release it during respiration, and decaying vegetation releases carbon dioxide too.^[11]

“More carbon dioxide always means more growth.” Plants also need water, sunlight and nutrients, and a plant lacking one of them will not grow, however plentiful the others are.^[8]

“A low NDVI means drought.” Unusually cold temperatures and clouds can also lower it.^[9]

“A greenness index counts how much plant there is.” One researcher put it this way: NDVI is “just a qualitative measurement of vegetation density”.^[10]

What this page does not cover

This page does not cover roots in detail, the life cycles of mosses, ferns and other spore plants, plant hormones and growth responses, crops and farming, the full classification of plants, or any count of plant species. The carbon figures above are dated and are not current values.

Going further

See Evolution, Ecology, Weather & Climate and Satellites.