286 



HOW CROPS GKOW. 



Fig;. 57. 



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 

 skin of the leaf. The size of 

 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. 5G 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 



