286 HOW CROPS GROW. 
usually of two curved cells, which are disposed toward 
each other nearly like the two sides of the letter O, or like 
the halves of an elliptical carriage-spring, (figs. 52 and 53). 
The opening between them 
is an actual orifice in the 
the orifice is, however, con- 
stantly changing, as the at- 
mosphere becomes drier or 
more moist, and as the sun- 
light acts more or less in- 
tensely on its surface. In 
moist air, they curve out- 
wards, and the aperture is 
enlarged; in dry air, they straighten and shut together 
like the springs of a heavily loaded carriage, and nearly 
or entirely close the entrance. The effect of strong light 
is to enlarge their orifices. 
In fig. 56 is represented a section through the shorter diameter of a 
pore on the under surface of a bean-leaf. The air-space within it is 
shaded black. Unlike the other epidermal cells, those of the leaf-pore 
contain grains of chlorophyll. 
Fig. 57 represents a portion of the epidermis of the upper surface of 
a potato-leaf, and fig. 58 a similar portion of the under surface of the same 
leaf, magnified 200 diameters. In both figures are seen the open pores 
between the semi-elliptical cells. The outline of the other epidermal 
cells is marked by irregular double lines. 
The round bodies in the cells of the 
pores are starch-grains, often present 
in these cells, when not existing in any 
other part of the leaf. . 
The stomata are with few ex- 
ceptions altogether wanting on 
the submerged leaves of aquatic 
plants. On floating leaves they 
occur, but only on the upper 
surface, Thus, as a rule, they 
are not found in contact with liquid water. On the other 
hand, they are either absent from, or comparatively few in 
Fig. 58, 
skin of the leaf. The size of 
