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A special variation on photosynthetic carbon fixation is found in maize (corn) and other tropical grasses is known as C4 photosynthesis. C4 plants are more efficient at fixing carbon dioxide under conditions of high light and temperature (e.g. Illinois farm fields in the middle of August!). One aspect of C4 plants is a specialized leaf anatomy, where mesophyll cells (light green) surround bundle-sheath cells (dark green). These in turn surround veins, the transport tubes that carry water into leaves and photosynthetic products away from them. The mesophyll cells contain a CO₂ "pump", which concentrate carbon-dioxide in the bundle-sheath cells, and eliminates the competing reaction of photorespiration.