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Cellulose acetate replicas were made from silicone rubber impressions of the abaxial surfaces of perennial ryegrass leaves. Apertures between 1 μ and 4 μ (mean 2 μ) were measured in replicas from leaves which had been exposed to conditions expected to close the stomata. This discrepancy is explained by observations of the ventral walls of guard cells in transverse sections of stomata, and by Araldite impressions of the cut ends of ryegrass leaves. These show that when a stoma is closed the ventral walls make contact below the paradermal walls of the guard cells and consequently a small depression is left between the cells. These depressions are similar in width to the apparent apertures in the cellulose replicas which may be wrongly interpreted as slightly open stomata. In some species the silicone impression method is incapable of giving an accurate measurement of stomatal aperture. Consequently, in attempts to relate such measurements to stomatal function or rate of transpiration it should be established that small apertures shown in cellulose acetate replicas are not the result of depressions between the external surfaces of guard cells in closed stomata.
The New Phytologist © 1969 New Phytologist Trust