40 



Mr. W. L. Balls. 



[Dec. 22, 



is slightly softened by the heat of the hand, and the leaf (I) merely pressed 

 upon it. When the leaf is to be exposed to strong sunlight, a white ring of 

 cardboard is centred over that portion in contact with the wax, at a distance of 

 a millimetre or two, by a three-point support. The chamber employed to 

 obtain the records here reproduced is shown in fig. 3a. On the suggestion of 

 Mr. Francis Darwin this has been modified to fig. 3b, in which the back is 

 made of glass, and so provides a nearly normal illumination to the lower side 

 of the leaf. 



When a leaf is to be left for many hours in the open air on such a chamber, 

 it is well to bind it lightly with wool in order to prevent the wind from 

 stripping it off, and so wasting battery power by leaving a free exit from the 

 gas-holder, besides interrupting the record. 



| 3. Records of Stomatal Movement. 



The appliance here described nominally records the velocity with which 

 air escapes through the stomata. The square root of this velocity (when 

 below a certain value) appears to represent more closely the actual stomatal 

 aperture.* Discussion of the difficult physical aspects of the matter is 

 beyond the purpose or the power of the present writer, but attention may be 

 drawn to one systematic error in the appliance. 



For low velocities, and until some such upper limit as a " fifteen-second 

 stroke " with the constants of pressure, capacity, and area given above, an 

 increase in the frequency of the stroke is probably directly proportional to 

 increases in mean stomatal aperture ; beyond some such upper limit, 

 however, the friction of the out- flowing air along the exit- tube and skin- 

 friction of the liquid on the gas-holder become noticeable, so that when 

 there is no leaf on the chamber the strokes still take about five seconds 

 each, which is the minimum time for leaves with fully-opened stomata. 

 The correction for this would have to be worked out empirically for each 

 appliance. 



Neglecting this for the present, the stomatograph can be used for 

 almost any kind of stomatal investigation, either in the laboratory or in the 

 field. Comparison of various leaves on the same plant can be effected 

 rapidly, by substituting one leaf after another on the chamber, as soon as 

 some five strokes of the pump have been recorded. One particular advantage 

 of the original porometer is that its results show the mean condition of 

 many hundreds of stomata in the experimental area; with the stomato- 

 graph this advantage can be extended to obtaining the mean of many 

 leaves on the same plant, by employing a number of chambers, all of which 

 * Darwin and Pertz, loc. cit. 



