ABSORPTION AND MOVEMENT OF WATER 145 



sary for photosynthesis. Other cells can work at a slower 

 rate and still accomplish as much, for, being independent of 

 light, they can work longer. These constitute some of the 

 reasons for the position and for the number of stomata. 



Stomata are sometimes called "breathing pores." They 

 . do admit oxygen, and whenever respiration exceeds photo- 

 synthesis (e. g. in the dark), the unused carbon-dioxide 

 may pass out through them. For these reasons they are 

 breathing pores, but this is neither their sole nor their main 

 function. Through open stomata water-vapor passes out- 

 ward and carbon-dioxide inward. Through the stomata on 

 the surface of the parts of the flower, little carbon-dioxide 

 passes inward, for these are not food-manufacturing organs. 

 On the contrary water-vapor and expired carbon-dioxide 

 pass outward. On such organs the stomata are essentially 

 organs for increasing and controlling transpiration.* 



A stoma consists fundamentally of a pair of epidermal 

 cells not completely connected together and, therefore, either 

 constantly separated from each other by a slit-like space, or 

 separated whenever the cells draw or are drawn apart. The 

 pair of incompletely connected cells — known as guard-cells — 

 are usually strikingly different in size, form, and contents 

 from other epidermal cells. The cells immediately adjacent 

 to them may also depart from the type of epidermal cells 

 both in appearance and in function. Such cells when present 

 are known as accessory or auxiliary cells, for they either sup- 

 plement the guard-cells in their task of opening and closing 

 the stoma, or they themselves open and close the stoma by 

 drawing the guard-cells apart or pushing them together. 

 Strictly speaking, the stoma is the slit-like opening (Spalt- 

 offbung) between these cells, just as the mouth is the cavity 

 opened and closed by the lips. 



The opening and closing of stomata are accomplished by 

 physical means only, but these means may or may not de- 

 pend upon the irritability of the living protoplasm of the 

 guard-cells and auxiliary cells. Fundamentally, the opening 



* Chester, Grace D. Bau und Function der Spaltoffnungen auf Blumen- 

 blattern und Antheren. Ber. d. Deutsch. Bot. Gesellsch., Bd. XV., 1897. 

 Older literature here cited. 

 10 



