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The CO2 diffusion pathway from the atmosphere surrounding the leaf to the chloroplast stroma is composed of several parts, where each part poses certain resistance to the CO2 flux. The CO2 diffusion from the sub-stomatal cavities to the stroma is limited due to mesophyll conductance (gm), which includes CO2 movement in gas, liquid and lipid phases. The mesophyll conductance is influenced by factors that are both internal (e.g. leaf anatomy and leaf age) and external (e.g. irradiance, temperature). Internal factors exhibit rather long-term responses of gm, while external factors are involved in short-term changes. For these rapid changes are probably responsible aquaporins. gm significantly influences the rate of net photosynthesis, almost in the same range as a stomatal conductance does. The limitation of photosynthesis given by mesophyll conductance is not fixed and varies among species and plant functional groups. Due to continual increase of CO2 concentration in the atmosphere, variability in gm can lead to enhanced competitive potencial of some plant species.

The global carbon cycle is in the focus of the scientists' interest, because understanding carbon sinks and their relationships can show the solutions of problems related with increasing CO2 concentration in the atmosphere in the future. I focused on the role of forests ecosystems in this carbon cycle. All forests cover almost one third of the Earth's land area. By photosynthesis they bind big part of atmospheric carbon to their biomass. Sink strength of forest can differ according to the type of biome, forest stand age and actual climatic conditions. Forest biomes can be generally divided according to the latitude to three groups: tropical forest biomes, temperate forests and boreal forests. The size of carbon stock decreases in this order. Other parameters, such as net primary production and respiration differ in dependence on the particular ecosystem. Tropical rain forests represent great carbon stock, but their deforestation causes massive C emissions back to the atmosphere. Boreal forests aren't considered as important carbon sink, but they influence local climate. Some speculations can also appear about the old forests. It is possible to find authors, who claims, that old forests are no more carbon sinks, so that their carbon balance is neutral. Many surveys prove that they bind less carbon...

The atmospheric carbon dioxide concentration have always been fluctuating. In the past [CO2] did not ever increase as quickly as in the last 200 years and apparently human activities such as fosil fuel burnig conribute significantly to this increase. Changes in stomatal frequency as a reaction on changes in [CO2] were observed on several species of living plants. It leads to investigation of stomatal fequency of herbarium samples and fossil plants. These estimates of stomatal frequency were consequently used to estimate levels of atmospheric [CO2] in the past. This study describes behavior of stomata in variable [CO2] and summarizes influence of environmental factors on stomatal patterning. The differences in responses between groups of plants of different evolutionary origin are considered. The advantages and disadvantages of selected stomatal parameters are discussed considering their suitability as tools for [CO2] estimate in the past.