136 



COMPENDIUM OF GENERAL BOTANY. 



widely separated divisions. Volume of front cavity and back cav- 

 ity variable ; large guard-cells, thin opposing walls. Principle : 

 a rubber tube thickened on 07ie side bends when pressure is in- 

 creased in its interior, while the thin wall becomes convex. The 

 thickened portions (ridges of the outer and inner apertures) of the 



Fig. 75. — Surface- view of an open {a) breathing-pore (stoma) and of one closed (ft). 

 (Diagramatic.) Illustrating types I and II. 



wall of the guard-cell wliich are nearest each other cause a curva- 

 ture of the guard-cells when the hydrostatic pressure of the interior 

 is increased. (By the presence of two such thickenings this cur- 

 vature is much more marked than it would be if only one such ridge 

 were present.) The thin areas of each guard-cell nearest the cen- 



FiG. IQ.—Uelleborus. Type II. 

 (After Schwendener.) 



tral passage may permit a hinge-joint movement of the thickened 

 ridges, or by mutual contact they may form a better means of clos- 

 ing the pore than do the thickened ridges. The chief mechanical 

 change involved is the greater expansibility of the thin distal walls 

 as compared with the thick proximal walls. 



