PERIODICITY OF STO^IATAL OPENING AND CLOSING 187 



the guard cells induces a decrease in the pH of their cell sap and a correlated 

 conversion of sugar to starch. The resulting diminution in the osmotic pres- 

 sure of the guard cells may lead to further loss of water from them into 

 adjacent epidermal cells. 



3. The Temperature Factor in the Opeiiing and Closing of Stomates. — 

 The results of various workers upon the influence of temperature on stomatal 

 behavior are not in agreement and divergent views have been expressed upon 

 the subject. According to Scarth (1932) relatively high temperatures (35-40° 

 C.) accentuate opening of the stomates of Zebrina pcndula in the light, and 

 prevent closure or even induce opening in the dark. Relatively low tempera- 

 tures (0-8° C.) prevent opening of the stomates of this species, even in the 

 light. There is some evidence that the pH of the cell sap of the guard cells 

 increases with rise in temperature and that this change in H-ion concentration 

 is accompanied by hydrolysis of starch to sugar. Presumably the reverse trans- 

 formation occurs at certain lower temperatures. 



Daily Periodicity of Stomatal Opening and Closing. — The stomates of 

 all species of plants which have been studied exhibit a more or less regular 

 daily periodicity of opening and closing, although the behavior of the stomates 

 upon plants of any one species varies, often markedly, depending upon the 

 pattern of the daily cycle of environmental factors. 



In the course of this book we shall have occasion to analyze the hour-to- 

 hour variations of a number of physiological processes occurring in plants. 

 Even in a given species the "daily periodicity" — as these hour-to-hour varia- 

 tions are termed — of any physiological process will vary greatly according to 

 environmental conditions. It will therefore be convenient to choose a definite 

 type of diurnal cycle of environmental factors as a reference standard in terms 

 of which to discuss daily variations in the rate of various processes. For this 

 purpose we shall select a "representative summer's day" as our criterion. We 

 shall consider this hypothetical day to be characterized by a clear sky, a soil 

 adequately well supplied with water {^i.e. at approximately the "field capacity," 

 Chap. XVI), and a maximum temperature of 30-35° C. (86-95° F.). Fur- 

 thermore we will assume that the daily march of the environmental factors of 

 solar radiation, air and soil temperatures, wind velocity, and atmospheric 

 humidity will be representative of a day upon which the conditions as defined 

 above prevail. In our subsequent discussion we shall refer to these as 

 "standard day conditions" (Fig. 48). Such environmental conditions will 

 actually be approximated on many summer days in temperate zone regions. 



It is important to realize that all of the stomates on a plant do not open 

 at the same time. Neither do all of them close at the same time. Under most 

 conditions, however, stomates probably are more nearly coincident in the time 



