52 



BOTANY 



PART 



FIG. 46. Epidermis with stomata from the lower surface 

 of the leaf of Helleborus niger. (x 120. After STRAS- 

 BURGER.) 



constitute the STOMATAL APPARATUS (Figs. 45 A, 46). The largest 

 stomata are found in grasses; thus in the wheat they measure 0'079 

 mm. in length by 0'039 mm. in breadth, while the pore itself is 

 0-038 mm. by O'OOT mm. 



The PORE interrupts the continuity of the epidermis. It is an 

 air-filled intercellular space opening below the epidermis into a large 



intercellular space (Fig. 

 45 B), which is spoken of 

 as the respiratory cavity 

 although it has nothing to 

 do with respiration. This 

 cavity is in communication 

 with the intercellular spaces 

 of the parenchyma. The 

 stomata are of great im- 

 portance to the plant, for 

 they place the system of 

 intercellular spaces, which 

 serves to ventilate the 

 tissues, in communication 

 with the external atmo- 

 sphere. This connection is 

 necessary on account of the difficulty with which gases pass across the 

 epidermis in order to renew the air in the intercellular spaces, and 

 especially to replace the carbon-dioxide as this is used up. On the 

 other hand, oxygen, which forms a considerable proportion of the air, 

 can usually penetrate into the plant in sufficient quantity through 

 the cuticle and the epidermal cells. 



The GUARD CELLS always contain chlorophyll and are character- 

 ised both by their shape and the mariner in which their walls are 

 thickened. This is best shown in transverse sections (Figs. 45 B, 

 47 B). There are usually an upper and a lower thickened band 

 on the side of the guard cell which faces the pore, the portion 

 of the wall between and the rest of the wall of the guard cell being 

 relatively thin (Fig. 45 B). This structure stands in relation to the 

 changes in form of the guard cells by means of which the size of the 

 pore is varied. The pore is closed by a diminution of the curvature of 

 the guard cells when there is danger of too great escape of moisture ; 

 while it is widely opened by increase in the volume of the guard cells 

 and consequently of their curvature at other times. The stomata 

 regulate the gaseous exchange and the transpiration. 



As the transverse section in Fig. 45 B shows, the thickening ridges project both 

 above and below the pore. There is thus an anterior chamber and a posterior 

 chamber in relation to the narrow region of the actual passage. The thickened 

 outer walls of the epidermal cells immediately adjoining the guard cells often 

 have a thinner portion which acts as a kind of hinge and enables the changes 



