320 



PHYSIOLOGY 



Fig. 628. — Cross section of stem of Myri- 

 ophyllum, with air canals. — From Part III. 



Stomata. — The aerating system of the terrestrial plants, and of water 

 plants not normally completely submersed, communicates with the at- 

 mosphere freely, because certain 

 cells of the epidermis, predeter- 

 mined by the mode of their de- 

 velopment, break apart through 

 the central portion of their last- 

 formed division wall. As imme- 

 diately beneath them an air space 

 of some size develops, this estab- 

 lishes a passage to the outer air. 

 These two crescentic cells of the 

 epidermis are the lips of a mouth- 

 like slit called a stoma; the two 

 lips are called guard cells (fig. 629). 

 The guard cells differ from other 

 epidermal cells in their crescentic 

 form and smaller size, and in having chloroplasts which are usually 

 absent from other epidermal cells. Their walls are also peculiarly and 

 unequally thickened (see also Part III, figs. 794-806). Their turgor 

 variations, the unequally thick walls, and their position with respect to 

 the adjacent cells make them change_ 

 shape, with increasing turgor becom- 

 ing more arcuate and with lessening 

 turgor straighter. The effect of these 

 changes is to widen or narrow the slit 

 between them, so making more free or 

 restricted the passage of gases either 

 by flow or diffusion. 



Size Eind number of stomata. — A 

 stoma is very minute; the area of the 

 pore when open, in thirty-seven sorts 

 of cultivated plants, averages 0.000092 

 sq. mm. But their great number on 

 those organs (such as leaves) in which 

 the admission and exit of gases is 

 most free, makes up for their small size. Both features will be grasped 

 better by this statement : in an area equal to that of the dot here 

 printed (• ), there are on the under side of the apple leaf over 1400 



Fig. 629. — Stoma of Sedum ; a,u,u, 

 first wall, cutting off mother cell of stoma ; 

 b, b, b, second; c, c, c, third; d, d, fourth; 

 e, e, final wall ; the latter, forming the two 

 guard cells, g, g, partially splits to form 

 the slit (i); i, 2, 3, subsidiary cells. 



