100 



BOTANY. 



the mother-cell is formed a median partition- wall forms 



in it, and gradually becomes separated into two plates, which 



eventually sepa- 

 rate and form a 

 pore through 

 the epidermis. 

 The two halves of 

 the mother-cell be- 

 come symmetrical- 

 ly rounded off into 

 semilunar or semi- 

 circular forms, 

 and constitute the 

 guard-cells before 

 mentioned. The 

 details of the fore- 

 going process in 

 one of its more 

 complex forms 

 are illustrated in 

 Fig. 91, A and B. 

 The splitting of 



the middle partition-wall of the mother-cell is shown in the 



successive sections (Fig. 92). 



132. In the light, under certain conditions of moisture 



and temperature, the 



guard-cells become 



curved away from each 



other in their central 



portions, thus opening 



the slit and allowing 



free communication 



between the external 



air and that in the in- 



, Fig. 90. Double *tomata from the under surface 

 tercellular Spaces and of the leaf ol 'Echlnocystls lobata. xSOO.-Fromi 



passages of the leaf. 



Fig. 89. Stomata from the under surface of the leaf of 

 Echmocystis lobata. s, s. stomata ; g, g, irregular epider- 

 mis-cells between the veins of the leaf ; v, elongated and 

 regular epidermis-cells over a vein. X 250. From a 

 drawing by J. C. Arthur. 



drawing by J. C. Arthur. 



(a) A superficial examination of stomata may be easily made by 

 stripping off the epidermis, and mounting it in water or alcohol. Good 

 sections of stomata are more difficult to make ; they may be obtained, 



