Chapter X — 197 — Loss and Retention 



nature where porometer figures have been plotted against observed stomatal areas ob- 

 tained either directly by use of a microscope on the living leaf or indirectly by Lloyd's 

 alcohol fixation method. The most recent attempt to calibrate the porometer is that 

 of Heath (1941) using the resistance porometer described by Gregory and Pearse 

 (1934). This instrument employs standard capillaries in series with leaf resistance. 

 In contrast to the older flow porometers, it can be used to calculate independently of 

 the pressure drop the resistance or conductance of the leaf area through which the 

 air flows. The conditions for the valid use of methods of calculating flow resistance 

 and the theoretical limitations involved can be found in Heath's paper. 



It has been observed (Williams, 1947) that the behavior of stomata under a 

 porometer cup cannot be regarded as normal at least so far as shock response is con- 

 cerned and consequently doubt is thrown upon observations made by use of the porom- 

 eter on stomatal response to light-dark stimuli. Further clarification of this problem 

 is needed. 



A very simple and rapid semiquantitative method which is well adapted to use in 

 the field where rough determinations are to be made is found in the infiltration method 

 (Molisch, 1912; Stein, 1912). If a drop of liquid of low surface tension and viscosity 

 is placed on a leaf with open stomata, it will penetrate through the stomata, filling the 

 intercellular spaces and giving a water-logged appearance. If a series of liquids of 

 differing surface tensions and viscosities is used, an approximate estimation of the 

 degree of stomatal opening can be made. In place of the series of liquids proposed by 

 Molisch (absolute alcohol, benzol, and xylol), other authors have used different 

 liquids. Stahl (1894) used paraffin oil, petroleum, and petroleum ether; Dietrich 

 (1925), xylol, petroleum, castor oil plus turpentine, and parafiin oil; Schorn (1929), 

 varying proportions of isobutyl alcohol and ethylene glycol. Weber (1923) employed 

 ammonium vapors which penetrate the leaf killing the cells and turning them brown. 



Certain other gross qualitative methods have been used to determine the opening 

 of stomata such as the use of cobalt chloride paper as a measure of transpiration, and 

 techniques in which the curling of strips of cellophane are used as a measure of humid- 

 ity. These methods are, however, of little value in studies of stomatal opening. 



Genetic Factors: — Shafer (1942), experimenting with excised leaves 

 of several inbred and hybrid corn seedhngs was unable to demonstrate any 

 differences in water loss between the various strains. Gyorffy (1941) 

 concluded from a comparative study of the osmotic values of a number of 

 plants that polyploids are more variable in osmotic pressure and hence 

 more adaptable to dry conditions than diploids. In grafts an advantageous 

 increase of osmotic value beyond the 2n level was observed in tetraploids. 

 Clark, Hecht, Curtis, and Shafer (1941) found that the stomata of 

 high yielding varieties of inbred and hybrid corn open earlier and close 

 later than those of low yielding strains. 



Stomatal Regulation of Transpiration: — The role of stomatal closure 

 in the control of transpiration has proved difficult to determine. Early 

 concepts following the demonstration of stomatal movement pictured 

 stomatal control as being complete except for cuticular transpiration. 

 Lloyd's (1908) work indicated that stomatal movement and transpiration 

 are not closely related. Loftfield (1921) claims these results to be in- 

 valid because of faulty technic. In work on alfalfa and several other plants, 

 Loftfield showed that stomatal movement and transpiration from cut 

 shoots paralleled each other quite closely. Stomatal movement in cut 

 shoots differed markedly from that in intact plants, hence Lloyd's compari- 

 son of stomatal opening on intact plants with transpiration of cut shoots 

 was hardly valid (see also Lloyd, 1912, 1913). Renner (1910) also 

 criticised Lloyd's conclusions. 



According to current opinion, it seems that when stomata are wide open, 

 transpiration rate depends upon factors governing evaporation. Among 

 these the diffusion pressure deficit of water as determined by tension in 



