WATER UTILIZATION BY TREES 67 



One of the difficulties has been to measure accurately the stomatal 

 openings. Of the various methods devise'd, four have been used 

 profitably by various investigators. These are (1) direct observation, 

 (2) porometers and stomatographs, (3) fixation, and (4) infiltration. 

 The direct observation of uninjured leaves under the microscope 

 would appear to be the simplest and most reliable method, but it is 

 generally difficult to apply, because of the small size of the stomata 

 and the opaqueness of the leaves. Such studies also must be carried 

 on in situ, which adds to the difficulty of the observations. Some 

 workers have tried to strip the epidermis from the leaves in the field 

 and observe directly with the microscope; but such manipulation runs 

 the danger of injuring the stomata in the process of stripping. 



The porometer, as devised by Darwin and Pertz (44) &nd improved 

 by Knight (124) Yosts on the principle that changes in stomatal width 

 can be registered by changes in the velocity of an air stream forced 

 through the leaf. It consists essentially of a T-tube, the lower end 

 of which rests in a vessel of water (or mercury). One of the arms of 

 the T-tube ends in an open rubber which can be clamped off; the 

 other ends in a small funnel which may be glued or attached to a leaf. 

 Through the open tube, water is drawn into the stem of the T-tube, 

 and the open tube is then clamped off. The pressure of the column 

 of water in the stem of the tube tends to draw in air through the leaf, 

 and the rate at which the water column drops is taken as an index, 

 of the condition of the stomata. If the stomata are completely! 

 closed the water column remains at the height to which it was originally 

 raised or changes its position very slowly, but if the stomata are wide 

 open, the water column falls rapidly. Furthermore Darwin (48) 

 checked the accuracy of the porometer by weighing, and showed that 

 in Prunus laurocerasus there is an agreement between the water losses 

 and the size of the stomatal opening; the water loss varies with the 

 square root of the porometer readings. 6 



In the fixation method as devised by Lloyd (139), the epidermis is 

 removed as quickly as possible and plunged into absolute alcohol, 

 fixing the stomata. This method has some of the disadvantages of 

 the direct method, and the whole operation must be done very rapidly. 

 It is also very important that a great many measurements be carried 

 out because of the sources of error involved. Other students have 

 used other fixatives, such as picric acid, but the principle is the same. 



The infiltration or injection method of Molisch (155) is simpler and 

 better adapted for use under natural conditions than some of the 

 previous methods, but it also has its shortcomings. This method 

 depends upon the fact that absolute alcohol placed on the epidermis 

 of a leaf penetrates only wide-open stomata, resulting in the appear- 

 ance of dark spots in the vicinity. If the stomata are nearly closed, 

 the alcohol cannot penetrate and infiltration does not occur; but other 

 liquids, such as benzol and xylol, can penetrate even partially closed 

 stomata. All that is needed for this method, then, are three bottles 

 containing absolute alcohol, benzol, and xylol, respectively, and 

 appropriate brushes or rods. First the alcohol is applied to the leaf 

 and, if it penetrates, the stomata are wide open; if benzol enters but 

 not the alcohol, the stomata are only partly open; if only xylol enters 

 the leaf, the stomata are nearly closed; while if none of the liquids 



« A self-registering modification of the porometer is Balls' stomatograph (8), for a description of which the 

 reader is referred to the original literature. 



