Role of light and carbon dioxide in the regulation of stomatal opening
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Stomata are functional units of the epidermis serving the exchange of gases between the intercellular spaces of the plant and its surrounding. They are especially common � and of characteristic shape � at the epidermis of the leaf�s underside of most species. Their development differs from plant group to plant group, but unequal cell divisions are always involved. A functional unit consists of the guard cells themselves that contain nearly always chloroplasts, and of their neighbouring subsidiary cells that are usually devoid of chloroplasts.
It is well-known that stomata open in a humid surrounding, and close when it is dry. J. HEDWIGwas the first researcher of the 18th century who understood that they work as �transpiration openings�. Today, it is spoken of their involvement in transpiration instead. It was thus not far-fetched to put the stomatas� mechanism back to changes of the turgor, and to classify them as turgor movements. This assumption was verified in plant-anatomical studies of the stomata (and especially of their cross-sections). The outer and the inner walls of many species are strongly thickened compared to the lateral walls. They put therefore up considerable esistance to every deformation. The wall between guard and subsidiary cell and the wall that makes up the opening are rather thin and are easily stretched. This asymmetry of the cell�s structure and of the wall thicknesses explains the directed movement caused by the turgor. The mechanics of the guard cells becomes thus understandable. The explanation how the movement is elicited and regulated was more difficult to find. Water proved to be an important, though not the only controlling factor of guard cell movements.
It is well-known that stomata open in a humid surrounding, and close when it is dry. J. HEDWIGwas the first researcher of the 18th century who understood that they work as �transpiration openings�. Today, it is spoken of their involvement in transpiration instead. It was thus not far-fetched to put the stomatas� mechanism back to changes of the turgor, and to classify them as turgor movements. This assumption was verified in plant-anatomical studies of the stomata (and especially of their cross-sections). The outer and the inner walls of many species are strongly thickened compared to the lateral walls. They put therefore up considerable esistance to every deformation. The wall between guard and subsidiary cell and the wall that makes up the opening are rather thin and are easily stretched. This asymmetry of the cell�s structure and of the wall thicknesses explains the directed movement caused by the turgor. The mechanics of the guard cells becomes thus understandable. The explanation how the movement is elicited and regulated was more difficult to find. Water proved to be an important, though not the only controlling factor of guard cell movements.
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