320 
PHYSIOLOGY 
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). 
FIG. 628. -Cross section of stem of Myri- The fd cells differ f rom other 
ophyllum, with air canals. From PART III. 
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 and 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, a, a, 
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 (5); i, 2, 3, subsidiary cells. 
