286 



HOW CROPS GROW. 



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 flu:. 50 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 

 iu 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 nquntic 

 pl:i nt s. On floating leaves they 

 occur, but only on the upper 

 surface. Thus, as a rule, they 



arc not fmun 1 in contact with liquid water. On the other 

 hand, they are either absent from, or comparatively few in 



