diffusion of water vapour and carbon dioxide through stomatal apertures. these developments, coupled with new methods for measuring the size of stomatal apertures, and the rate of photosynthesis have enabled precise description of the rates of gas exchange between plants and the atmosphere. innovations in statistical analysis by ronald fisher, frank yates and others at rothamsted experimental station facilitated rational experimental design and data analysis in botanical research. the discovery and identification of the auxin plant hormones by kenneth v. thimann in 1948 enabled regulation of plant growth by externally applied chemicals. frederick campion steward pioneered techniques of micropropagation and plant tissue culture controlled by plant hormones. the synthetic auxin 2,4-dichlorophenoxyacetic acid or 2,4-d was one of the first commercial synthetic herbicides. micropropagation of transgenic plants 20th century developments in plant biochemistry have been driven by modern techniques of organic chemical analysis, such as spectroscopy, chromatography and electrophoresis. with the rise of the related molecular-scale biological approaches of molecular biology, genomics, proteomics and metabolomics, the relationship between the plant genome and most aspects of the biochemistry, physiology, morphology and behaviour of plants can be subjected to detailed experimental analysis. the concept originally stated by gottlieb haberlandt in 1902 that all plant cells are totipotent and can be grown in vitro ultimately enabled the use of genetic engineering experimentally to knock out a gene or genes responsible for a specific trait, or to add genes such as gfp that report when a gene of interest is being expressed. these technologies enable the biotechnological use of whole plants or plant cell cultures grown in bioreactors to synthesise pesticides, antibiotics or other pharmaceuticals, as well as the practical application of genetically modified crops designed for traits such as improved yield. modern morphology recognises a continuum between the major morphological categories of root, stem (caulome), leaf (phyllome) and trichome. furthermore, it emphasises structural dynamics. modern systematics aims to reflect and discover phylogenetic relationships between plants. modern molecular phylogenetics largely ignores morphological characters, relying on dna sequences as data. molecular analysis of dna sequences from most families of flowering plants enabled the angiosperm phylogeny group to publish in 1998 a phylogeny of flowering plants, answering many of the questions about relationships among angiosperm families and species. the theoretical possibility of a practical method for identification of plant species and commercial varieties by dna barcoding is the subject of active current research. scope and importance botany involves the recording and description of plants, such as this herbarium specimen of the lady fern athyrium filix-femina. the study of plants is vital because they underpin almost all animal life on earth by generating a large proportion of the oxygen and food that provide humans and other organisms with aerobic respiration with the chemical energy they need to exist. plants, algae and cyanobacteria are the major groups of organisms that carry out photosynthesis, a process that uses the energy of sunlight to convert water and carbon dioxide into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells. as a by-product of photosynthesis, plants release oxygen into the atmosphere, a gas that is required by nearly all living things to carry out cellular respiration. in addition, they are influential in the global carbon and water cycles and plant roots bind and stabilise soils, preventing soil erosion. plants are crucial to the future of human society as they provide food, oxygen, biochemicals, and products for people, as well as creating and preserving soil. historically, all living things were classified as either animals or plants and botany covered the study of all organisms not considered animals. botanists examine both the internal functions and processes within plant organelles, cells, tissues, whole plants, plant populations and plant communities. at each of these levels, a botanist may be concerned with the classification (taxonomy), phylogeny and evolution, structure (anatomy and morphology), or function (physiology) of plant life. the strictest definition of "plant" includes only the "land plants" or embryophytes, which include seed plants (gymnosperms, including the pines, and flowering plants) and the free-sporing cryptogams including ferns, clubmosses, liverworts, hornworts and mosses. embryophytes are multicellular eukaryotes descended from an ancestor that obtained its energy from sunlight by photosynthesis. they have life cycles with alternating haploid and diploid phases. the sexual haploid phase of embryophytes, known as the gametophyte, nurtures the developing diploid embryo sporophyte within its tissues for at least part of its life, even in the seed plants, where the gametophyte itself is nurtured by its parent sporophyte. other groups of organisms that were previously studied by botanists include bacteria (now studied in bacteriology), fungi (mycology) – including lichen-forming fungi (lichenology), non-chlorophyte algae (phycology), and viruses (virology). however, attention is still given to these groups by botanists, and fungi (including lichens) and photosynthetic protists are usually covered in introductory botany courses. palaeobotanists study ancient plants in the fossil record to provide information about the evolutionary history of plants. cyanobacteria, the first oxygen-releasing photosynthetic organisms on earth, are thought to have given rise to the ancestor of plants by entering into an endosymbiotic relationship with an early eukaryote, ultimately becoming the chloroplasts in plant cells. the new photosynthetic plants (along with their algal relatives) accelerated the rise in atmospheric oxygen started by the cyanobacteria, changing the ancient oxygen-free, reducing, atmosphere to one in which free oxygen has been abundant for more than 2 billion years. among the important botanical questions of the 21st century are the role of plants as primary producers in the global cycling of life's basic ingredients: energy, carbon, oxygen, nitrogen and water, and ways that our plant stewardship can help address the global environmental issues of resource management, conservation, human food security, biologically invasive organisms, carbon sequestration, climate change, and sustainability. human nutrition further information: human nutrition the food we eat comes directly or indirectly from plants such as rice. virtually all staple foods come either directly from primary production by plants, or indirectly from animals that eat them. plants and other photosynthetic organisms are at the base of most food chains because they use the energy from the sun and nutrients from the soil and atmosphere, converting them into a form that can be used by animals. this is what ecologists call the first trophic level. the modern forms of the major staple foods, such as hemp, teff, maize, rice, wheat and other cereal grasses, pulses, bananas and plantains, as well as hemp, flax and cotton grown for their fibres, are the outcome of prehistoric selection over thousands of years from among wild ancestral plants with the most desirable characteristics. botanists study how plants produce food and how to increase yields, for example through plant breeding, making their work important to humanity's ability to feed the world and provide food security for future generations. botanists also study weeds, which are a considerable problem in agriculture, and the biology and control of plant pathogens in agriculture and natural ecosystems. ethnobotany is the study of the relationships between plants and people. when applied to the investigation of historical plant–people relationships ethnobotany may be referred to as archaeobotany or palaeoethnobotany. some of the earliest plant-people relationships arose between the indigenous people of canada in identifying edible plants from inedible plants. this relationship the indigenous people had with plants was recorded by ethnobotanists. plant biochemistry plant biochemistry is the study of the chemical processes used by plants. some of these processes are used in their primary metabolism like the photosynthetic calvin cycle and crassulacean acid metabolism. others make specialised materials like the cellulose and lignin used to build their bodies, and secondary products like resins and aroma compounds. plants make various photosynthetic pigments, some of which can be seen here through paper chromatography xanthophylls chlorophyll a chlorophyll b plants and various other groups of photosynthetic eukaryotes collectively known as "algae" have unique organelles known as chloroplasts. chloroplasts are thought to be descended from cyanobacteria that formed endosymbiotic relationships with ancient plant and algal ancestors. chloroplasts and cyanobacteria contain the blue-green pigment chlorophyll a. chlorophyll a (as well as its plant and green algal-specific cousin chlorophyll b) absorbs light in the blue-violet and orange/red parts of the spectrum while reflecting and transmitting the green light that we see as the characteristic colour of these organisms. the energy in the red and blue light that these pigments absorb is used by chloroplasts to make energy-rich carbon compounds from carbon dioxide and water by oxygenic photosynthesis, a process that generates molecular oxygen (o2) as a by-product. the calvin cycle (interactive diagram) the calvin cycle incorporates carbon dioxide into sugar molecules. rubisco carbon fixation reduction 3-phosphoglycerate 3-phosphoglycerate carbon dioxide 1,3-biphosphoglycerate glyceraldehyde-3-phosphate (g3p) inorganic phosphate ribulose 5-phosphate ribulose-1,5-bisphosphate edit · source image the light energy captured by chlorophyll a is initially in the form of electrons (and later a proton gradient) that's used to make molecules of atp and nadph which temporarily store and transport energy. their energy is used in the light-independent reactions of the calvin cycle by the enzyme rubisco to produce molecules of the 3-carbon sugar glyceraldehyde 3-phosphate (g3p). glyceraldehyde 3-phosphate is the first product of photosynthesis and the raw material from which glucose and almost all other organic molecules of biological origin are synthesised. some of the glucose is converted to starch which is stored in the chloroplast. starch is the characteristic energy store of most land plants and algae, while inulin, a polymer of fructose is used for the same purpose in the sunflower family asteraceae. some of the glucose is converted to sucrose (common table sugar) for export to the rest of the plant. unlike in animals (which lack chloroplasts), plants and their eukaryote relatives have delegated many biochemical roles to their chloroplasts, including synthesising all their fatty acids, and most amino acids. the fatty acids that chloroplasts make are used for many things, such as providing material to build cell membranes out of and making the polymer cutin which is found in the plant cuticle that protects land plants from drying out. plants synthesise a number of unique polymers like the polysaccharide molecules cellulose, pectin and xyloglucan from which the land plant cell wall is constructed. vascular land plants make lignin, a polymer used to strengthen the secondary cell walls of xylem tracheids and vessels to keep them from collapsing when a plant sucks water through them under water stress. lignin is also used in other cell types like sclerenchyma fibres that provide structural support for a plant and is a major constituent of wood. sporopollenin is a chemically resistant polymer found in the outer cell walls of spores and pollen of land plants responsible for the survival of early land plant spores and the pollen of seed plants in the fossil record. it is widely regarded as a marker for the start of land plant evolution during the ordovician period. the concentration of carbon dioxide in the atmosphere today is much lower than it was when plants emerged onto land during the ordovician and silurian periods. many monocots like maize and the pineapple and some dicots like the asteraceae have since independently evolved pathways like crassulacean acid metabolism and the c4 carbon fixation pathway for photosynthesis which avoid the losses resulting from photorespiration in the more common c3 carbon fixation pathway. these biochemical strategies are unique to land plants. medicine and materials phytochemistry is a branch of plant biochemistry primarily concerned with the chemical substances produced by plants during secondary metabolism. some of these compounds are toxins such as the alkaloid coniine from hemlock. others, such as the essential oils peppermint oil and lemon oil are useful for their aroma, as flavourings and spices (e.g., capsaicin), and in medicine as pharmaceuticals as in opium from opium poppies. many medicinal and recreational drugs, such as tetrahydrocannabinol (active ingredient in cannabis), caffeine, morphine and nicotine come directly from plants. others are simple derivatives of botanical natural products. for example, the pain killer aspirin is the acetyl ester of salicylic acid, originally isolated from the bark of willow trees, and a wide range of opiate painkillers like heroin are obtained by chemical modification of morphine obtained from the opium poppy. popular stimulants come from plants, such as caffeine from coffee, tea and chocolate, and nicotine from tobacco. most