TISSUES AND SIMPLE ORGANS. 



135 



{a) The Structure and Function of Breathing -po7'es {Stomata). 



(With Figs. 74-79.) 



Immediately below the guard-cells there is a large intercellular 

 space called the air-chamher into which the intercellular canals of 

 the surrounding tissue lead. The space at v (Fig, 74) is known as 

 " front cavity " (Yorhof, entrance), the one at A as " back cavity " 

 (Hinterhof) ; between them lies the central passage (Centralspalte) ; 

 s are the two guard-cells ; g the cuticular joint (Hautgelenk) ; a 



Fig. 74. — Vertical sectiou through the stoma of Amaryllis formosissima. Type 1. 

 «, Air-chamber. (After Schwendener.) 



the air-cavity. As a rule, the cuticle covers the wall of the 

 guard-cell about the front cavity, and sometimes extends even to 

 the air-cavity, as shown by Schwendener. The consideration of 

 the mechanics of breathing-pores reveals one of the most interest- 

 ing accomplishments of modern teleological, or, better, anatomical- 

 physiological, investigations. We shall now briefly consider three 

 main types recognized as such owing to essential peculiarities: 1. 

 Aniaryllis-tji^e, 2. Ilellehoi'us-tjpe, 3, Gramineous type (ac- 

 cording to Schwendener's investigations). 



First type: Amaryllis formosissi^na and many other plants of 



