The stomata of Phyllitis scolopendrium differ from those of angiosperms in that the guard cells contain many chloroplasts, comparable in size and appearance with those of the mesophyll. Experiments are described showing that the stomata of this fern respond to light and darkness and changes in carbon dioxide concentration in the same way as those of angiosperms. The stomata opened widely in conditions in which there could be no net photosynthetic carbon dioxide fixation, and it is therefore concluded that substances produced from carbon dioxide by photosynthesis do not play a major role in the opening mechanism and possibly no direct role at all.
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