1911.] New Method of Estimating the Aperture of Stomata. 137 



vitiated by want of precise knowledge as to the stomatal aperture during the 

 course of the enquiry.* 



Hitherto it has also been found difficult to test assumption (2), i.e., that 

 transpiration is a function of stomatal aperture, because we had no really 

 trustworthy method of estimating that aperture in the living leaf. Stahl's 

 cobalt methodf and the horn hygroscopej are not free from Lloyd' s§ objection 

 to them — that they indicate variations in the yield of water-vapour which 

 need not necessarily be dependent on changes in stomatal aperture. The 

 point we are discussing is the subject matter of Lloyd's book on stomata. 

 His method of observation is to strip the epidermis from the living leaf and 

 plunge it at once into absolute alcohol. He asserts that specimens so 

 prepared exhibit under the microscope the condition of the stoma as it 

 was in life. The result of a series of careful measurements, compared with 

 records of transpiration, is to convince Lloyd that the aperture of the stoma 

 is not the dominant factor, and that transpiration is, on the contrary, regulated 

 by some unknown properties of the plant. He does not, I think, tell us what 

 he suspects these unknown properties to be, but it is not difficult to imagine 

 internal interference with the loss of water. 



Although we recognise the value of Lloyd's work, we are not convinced by it ; 

 we believe that a much more intimate knowledge of the living stoma and its 

 movements would be necessary to prove his contention|| that " the regulatory 

 function is almost nil." With a view to testing the question we have 

 designed an instrument which we propose to call a porometer.1T The idea 

 is to estimate changes in the stomata by recording the change in the velocity 

 of a current of air drawn through them in the living leaf. The construction 

 is shown in the following figure in a diagrammatic form. 



A glass chamber (C) bearing a broad flange is cemented to the stomatal 

 surface of a leaf (L). A rubber tube connects C with a tube (T), one limb of 

 which is graduated and dips into the vessel of water (Y). The other limb 

 ends in a tube controlled by a clamp (M). By applying suction (as indicated 

 by the arrow) and then closing the clamp M, a column of water is raised to A. 



* We hope to publish before long a method of demonstrating the dependence of 

 transpiration on relative humidity, of which some account was given at the Sheffield 

 meeting of the British Association in 1910, when some of the experiments given in the 

 present paper were also described. 



t Stalil ('94). (See Bibliography, p. 154, infra.). 



+ F. Darwin ('98). 



§ Lloyd ( ! 08). 



|| Lloyd ('08), p. 45 ; elsewhere (pp. 35 and 44) he allows that the extreme limits of 

 transpiration are fixed by the stomata. 



IT It is on the same general principle as N. C. J. Miiller's apparatus, which never came 

 into use owing to its cumbrous make. See Miiller, N. ('73). 



