220 TEXTBOOK OF BOTANY 



sugar, while a slight increase in acidity results in a condensation of sugar 

 to starch. These changes in acidity and their consequences may be 

 brought about experimentally by immersing pieces of the epidermis in 

 chemical solutions differing in acidity. Under these experimental condi- 

 tions the opening and closing of the stomates may be brought about at 

 any time of day or night. 



Under natural conditions the stomates of many plants are open during 

 the day and closed at night. Certain oxidation-reduction processes in- 

 itiated in the guard cells by light result in a decrease in their acidity; 

 the starch in them is soon hydrolyzed to sugar, which dilutes the water 

 in the guard cells below that of the surrounding cells. The consequent 

 osmosis, increase in cell turgor, and swelling of the guard cells result 

 in the opening of the stomate. 



The stomates may also open in darkness if the temperature is high. 

 The effect of high temperature on the relative rates of processes in the 

 guard cells reduces the acidity sufficiently to initiate the chain of proc- 

 esses that results in the opening of stomates. The decreased turgor in 

 the guard cells of leaves with reduced water content may result in the 

 closing of stomates during the day. During droughts they may remain 

 open but an hour or two in the morning. The behavior of guard cells is 

 not the same in all species of plants. In a few plants the stomates are 

 open both day and night. 



Summary. In many of the examples cited in this chapter the diffusion 

 of water may be correctly referred to as osmosis in contrast to the move- 

 ment of materials that should be referred to simply as diffusion. Several 

 familiar plant phenomena that are dependent largely upon osmosis and 

 cell turgor have been cited and briefly described so that the reader may 

 check his own observations and interpretations on the basis of the facts 

 involved. 



Some of the structures, movements, and curvatures described are un- 

 doubtedly advantageous to the plant and may be important in its sur- 

 vival in nature. Others belong to that great group of plant phenomena 

 that evolve through the ages and survive through heredity without being 

 either essential or destructive to the species in which they occur. The 

 movements and curvatures, whether advantageous or not, are the con- 

 sequences of cell wall extension and of the energy of molecular motion 

 in diffusion, osmosis, and cell turgor. Since we do not personify these 

 processes and their consequences when they occur in non-living sys- 

 tems, there seems to be no good reason why we should personify them 

 when they occur in living systems. 



