1911.] New Method of Estimating the Aperture of Stomata. 139 



§ 2. Difficulties. 



The chief difficulty encountered was to find a method of fixing the chamber 

 to the leaf which should be non-injurious and should make an air-tight joint. 

 After many trials we conclude that ordinary glue is the best medium : it 

 adheres well both to the leaf and to the glass flange, and is not injurious. 

 The best way is to let the glue get fairly cool (say 30° C), and paint it 

 thickly on to the flange of the chamber, which is then gently pressed on to 

 the stomatal surface of the (inverted) leaf and clamped in that position, the 

 leaf being supported on a horizontal glass plate (not shown in fig. 1). 

 Another method is to cut out a washer (i.e., a perforated disc) from a layer of 

 20-25 per cent, gelatine about a centimetre in thickness, and to press the 

 chamber firmly down on the washer, and clamp it in that position. Here, 

 again, the leaf has the stomatal side upwards, and is supported by a horizontal 

 glass plate. This method is best for tough leaves, e.g., Ficns elastica, 

 P. laurocerasus, ivy (H. helix), etc., which do not appear to be injured in spite 

 of being compressed between the gelatine and the supporting glass plate. 

 With care, however, delicate leaves may safely be treated in the same way. 

 In the earlier experiments gelatine with a percentage of glycerine was used ; 

 this, however, is injurious. The same may be said of vaseline and other 

 greasy substances which were employed at an early stage of the inquiry. 



§ 3. The Porometer compared with other methods, i.e., the Cobalt Test, the Horn 

 Hygroscope, and Lloyd's Microscopic Method. 

 The porometric method is a direct one, and must therefore be classed with 

 the microscopic method, since both are absolutely independent of transpira- 

 tion,* whereas the two hygroscopic methods are indirect, and alterations in 

 stomatal aperture cannot be inferred with certainty from the observations in 

 •question. 



The porometer shares with the last-named methods the great advantage of 

 being continuous, that is to say, it allows of prolonged observation of a given 

 leaf. Lloyd's method entails the sacrifice of a leaf for each observation. 

 Moreover, his measurements are not actual observations on living tissue, 

 whereas the porometer and the hygroscopic methods have this merit. 



A striking point about the porometer is its great range. Thus the rate of 

 air-flow in an illuminated leaf may be as much as 400 times as rapid as the 

 flow through the same leaf in darkness. The porometer, being more delicate 

 than the hygroscopic methods, confirms the observations made some years 



* All that is here meant is that the flow of air through the stomata is not in any way 

 influenced by the amount of water vapour diffusing through the same openings. 



