PERIODICITY OF STOAIATAL OPENING AND CLOSING 189 



and finally falls to a virtually zero value at approximately the termination 

 of the daylight period. The stomatcs apparently behave in this M^ay when 

 a water deficit develops in the leaves somewhat earlier in the day than under 

 "standard day" conditions. Partial closure of the stomates results during 

 the morning hours. The resultant reduction in their diffusive capacity per- 

 mits an increase in the water content of the leaf, and for a time the stomatal 

 apertures again widen. Subsequently the water deficit of the leaf increases 

 again, due to increased transpirational loss, and the stomates enter upon a 

 second cycle of closing which continues throughout the remainder of the day- 

 light period. 



When the soil water supply is distinctly inadequate the stomates usually 

 open incompletely and seldom remain open for the entire daylight period. 

 Although with the advent of daylight the light factor favors stomatal open- 

 ing, especially on clear daj's, the water content of the leaf is so low that 

 opening is seldom complete. Furthermore the effect of the water factor 

 usually begins to predominate over the light factor relatively early in the day, 

 and stomatal closure may be complete by mid-day or even sooner. During 

 prolonged droughts the stomates generally close progressively earlier and 

 earlier each day and ultimately matinal opening may cease almost entirely. 



On cloudy or rainy days, especially if the temperature is relatively low, 

 the stomates of most species open less completely than on clear days when 

 the soil is well supplied with water. This is due principally to the ineffective- 

 ness of the light factor in inducing stomatal opening under such conditions. 

 Hence opening of the stomates is incomplete and they do not remain open as 

 long under such meteorological conditions as on a "standard day." 



Nocturnal opening of the stomates has been reported for a number of 

 species of plants (Loftfield, 1921, Desai, 1937, and others) but the condi- 

 tions which result in this type of stomatal behavior are not clearly under- 

 stood. Apparently it may be brought about by different combinations of 

 environmental conditions and it seems likely that the conditions leading to 

 nocturnal opening of the stomates may differ according to species. In the 

 Norway spruce {Picea excelsa), for example, the stomates remain open during 

 the night after a cloudy day on which relatively high temperatures have pre- 

 vailed (Stalfelt, 1929.) There are some indications that high temperatures 

 and a deficient water supply favor nocturnal opening of the stomates in some 

 species. Scarth ct a!. (1933) have shown that a reduced partial pressure 

 of oxygen in the atmosphere leads to stomatal opening in the dark. They 

 suggest that nocturnal opening may be due to a reduced partial pressure of 

 oxygen in the intercellular spaces resulting from night respiration. Finally 



