CELLULAR STRUCTURE OF LEAVES 



247 



A surface view of a small portion of epidermis stripped off and 

 highly magnified is shown in Figure 230. The epidermal cells in 

 this view are irregular in shape, but so closely fitted together 

 that no openings occur except through the stomata. A stoma 

 (singular of stomata) is a definite structure, consisting of two 

 curved cells, known as guard cells, which are so fitted together 

 as to enclose a slit-like opening. The guard cells are so named 

 because they regulate the size of the opening. Some plants, 

 such as the Grasses of which Corn is 

 a familiar example, have a peculiar 

 type of guard cells as Figure 231 

 shows. In this case the guard cells 

 are enlarged at the ends, and re- 

 semble dumbbells in shape. How- 

 ever, this difference in shape seems 

 to have nothing to do with their 

 behavior, for they open and close 

 their slit-like opening just as the 

 ordinary type of guard cells is able 

 to do. 



By changes taking place within 

 the guard cells, the stomata are 

 opened and closed, but the causes 

 of such changes are not definitely 



known. The guard cells have 



IT , , j ji -j FIG. 231. A much enlarged 



chloroplasts, and there is consider- gurface ^ of ^ epidermig of 



able evidence that the chloroplasts Corn, showing one stoma. g, 

 have something to do with bringing guard cells; t, slit-like opening; 

 about these changes. Since chloro- e, epidermal cells. The chloro- 

 plasts make sugar and have the plasts are in the ends of the guard 

 power to transform sugar into starch 



or starch into sugar, it is evident that they can alter the concentra- 

 tion of the sugar in the cell sap and in this way alter the turgor pres- 

 sure of the guard cells. For example, if the chloroplasts of the 

 guard cells manufacture much sugar which is allowed to concen- 

 trate in the cell sap, then by the principle of osmosis the guard 

 cells draw in water forcibly and develop a high internal pressure 

 which tends to expand them and alter their shape. On the other 

 hand, if the chloroplasts remove the sugar from the cell sap by 

 changing it into starch, which is insoluble, the result may be 



