Query: Plant stomata show higher and more variable CO2 levels

Relevant passages:
Stoma <=SEP=> In botany , a stoma ( plural `` stomata '' ) , also called a stomate ( plural `` stomates '' ) ( from Greek στόμα , `` mouth '' ) , is a pore , found in the epidermis of leaves , stems , and other organs , that facilitates gas exchange . The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the stomatal opening .   The term is usually used collectively to refer to the entire stomatal complex , consisting of the paired guard cells and the pore itself , which is referred to as the stomatal aperture . Air enters the plant through these openings by gaseous diffusion , and contains carbon dioxide and oxygen , which are used in photosynthesis and respiration , respectively . Oxygen produced as a by-product of photosynthesis diffuses out to the atmosphere through these same openings . Also , water vapor diffuses through the stomata into the atmosphere in a process called transpiration .   Stomata are present in the sporophyte generation of all land plant groups except liverworts . In vascular plants the number , size and distribution of stomata varies widely . Dicotyledons usually have more stomata on the lower surface of the leaves than the upper surface . Monocotyledons such as onion , oat and maize may have about the same number of stomata on both leaf surfaces . In plants with floating leaves , stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely . Most tree species have stomata only on the lower leaf surface . Leaves with stomata on both the upper and lower leaf are called amphistomatous leaves ; leaves with stomata only on the lower surface are hypostomatous , and leaves with stomata only on the upper surface are epistomatous or hyperstomatous . Size varies across species , with end-to-end lengths ranging from 10 to 80 µm and width ranging from a few to 50 µm .

Proxy (climate) <=SEP=> In the study of past climates ( `` paleoclimatology '' ) , climate proxies are preserved physical characteristics of the past that stand in for direct meteorological measurements and enable scientists to reconstruct the climatic conditions over a longer fraction of the Earth 's history . Reliable global records of climate only began in the 1880s , and proxies provide the only means for scientists to determine climatic patterns before record-keeping began .   Examples of proxies include ice cores , tree rings , sub-fossil pollen , boreholes , corals , lake and ocean sediments , and carbonate speleothems . The character of deposition or rate of growth of the proxies ' material has been influenced by the climatic conditions of the time in which they were laid down or grew . Chemical traces produced by climatic changes , such as quantities of particular isotopes , can be recovered from proxies . Some proxies , such as gas bubbles trapped in ice , enable traces of the ancient atmosphere to be recovered and measured directly to provide a history of fluctuations in the composition of the Earth 's atmosphere . To produce the most precise results , systematic cross-verification between proxy indicators is necessary for accuracy in readings and record-keeping .   Proxies can be combined to produce temperature reconstructions longer than the instrumental temperature record and can inform discussions of global warming . The distribution of proxy records , just like the instrumental record , is not at all uniform , with more records in the northern hemisphere .

Photosynthesis <=SEP=> Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms ' activities ( energy transformation ) . This chemical energy is stored in carbohydrate molecules , such as sugars , which are synthesized from carbon dioxide and water -- hence the name photosynthesis , from the Greek φῶς , phōs , `` light '' , and σύνθεσις , synthesis , `` putting together '' . In most cases , oxygen is also released as a waste product . Most plants , most algae , and cyanobacteria perform photosynthesis ; such organisms are called photoautotrophs . Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth 's atmosphere , and supplies all of the organic compounds and most of the energy necessary for life on Earth .   Although photosynthesis is performed differently by different species , the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments . In plants , these proteins are held inside organelles called chloroplasts , which are most abundant in leaf cells , while in bacteria they are embedded in the plasma membrane . In these light-dependent reactions , some energy is used to strip electrons from suitable substances , such as water , producing oxygen gas . The hydrogen freed by the splitting of water is used in the creation of two further compounds that act as an immediate energy storage means : reduced nicotinamide adenine dinucleotide phosphate ( NADPH ) and adenosine triphosphate ( ATP ) , the `` energy currency '' of cells .   In plants , algae and cyanobacteria , long-term energy storage in the form of sugars is produced by a subsequent sequence of reactions called the Calvin cycle ; some bacteria use different mechanisms , such as the reverse Krebs cycle , to achieve the same end . In the Calvin cycle , atmospheric carbon dioxide is incorporated into already existing organic carbon compounds , such as ribulose bisphosphate ( RuBP ) . Using the ATP and NADPH produced by the light-dependent reactions , the resulting compounds are then reduced and removed to form further carbohydrates , such as glucose .   The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide , rather than water , as sources of electrons . Cyanobacteria appeared later ; the excess oxygen they produced contributed directly to the oxygenation of the Earth , which rendered the evolution of complex life possible . Today , the average rate of energy capture by photosynthesis globally is approximately 130 terawatts , which is about three times the current power consumption of human civilization .  Photosynthetic organisms also convert around 100 -- 115 thousand million metric tonnes of carbon into biomass per year .