The guard cells ofTulipa gesnerianaL. were found to contain both chlorophyllsaandb. The total chlorophyll content was estimated to be of the order of 0.5ensp;times;ensp;10minus;12ensp;gm, per cell. By exposure to radioactive carbon dioxide followed by microautoradiography, selective carbon dioxide uptake in the light was demonstrated in the stomatal cells ofAllium,Hordeum,Nicotiana,Sedum,Tradescantia,Tulipa, andVicia, A less intense uptake was observed in darkness withAllium,Sedum,Tulipa, andVicia. The rate of guard-cell photosynthesis was estimated to be of the order of 0.02ensp;times;ensp;10minus;12ensp;M. carbon dioxide per cell per hour at 500ensp;ft-c. and in 0.04 (v/v) carbon dioxide. It was concluded that this rate was too low to account for the increase in osmotic potential caused by illumination.
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