Experimental researches on vegetable assimilation and respiration. XVIII.—The relation between stomatal opening and assimilation.—A critical study of assimilation rates and porometer rates in leaves of Cherry Laurel

Abstract In a previous communication it was shown that there is, in Cherry Laurel leaves, at “limiting” concentrations of CO2, a marked diurnal rhythm of apparent assimilation. In spite of the maintenance of a constant high light intensity, the assimilation rate falls to very low values during the night but rises again in the morning. At high concentrations of CO2, such that light, instead of CO2, is limiting the rate, there is no diurnal rhythm, the light limited value of assimilation being steadily maintained for more than 24 hours. There is, in addition to the diurnal rhythm, a seasonal rhythm of assimilation—the pitch or level of the diurnal assimilation curves being higher in November than in July, August and September. Also the level of assimilation, for a given “limiting” concentration of CO2, is higher at high light intensity than at low. It was suggested that these variations in assimilation were largely due to variations in stomatal opening. The work on porometer rates and assimilation rates described in the present paper was undertaken in order to test this hypothesis and to explore, as far as was possible by such means, the relation between stomatal opening and assimilation rate.