VEGETATIVE OKGANS OF PLANTS. 309 



The epidermis, while often smooth, is frequently beset 

 with hairs or glands, as seen in the figure. These are 

 variously shaped cells, sometimes empty, sometimes, as 

 in the nettle, filled with an irritating liquid. 



Leaf-Pores. — The epidermis of the mature leaf is pro- 

 vided with a vast number of "breathing pores," or s^oma^rt, 

 by means of which the intercellular spaces in the interior 

 of the leaf are brought into direct communication with 

 the outer atmosphere. Each of these stomata consists 

 usually of two curved guard-cells, which are disposed 



toward each other like the 

 halves of an elliptical car- 

 riage-spring. (Figs. 52 and 

 53.) Tlie opening between 

 them is an actual orifice in 

 the skin of the leaf. The 

 size of the orifice is, how- 

 ever, constantly changing, 

 as the atmosphere becomes 

 drier or more moist, and as 

 '^' ■ the sunlight acts more or 



less intensely on its surface. In strong light they curve 

 outwards, and the aperture is enlarged ; in darkness they 

 straighten and shut together, like the springs of a heavily- 

 loaded carriage, and nearly or entirely close the entrance.' 

 The effect of water usually is to 

 close 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-pores contain chlorophyll 

 granules. 



Fig. 57 represents a portion of the epi- 

 dermis of the upper surface of a potato- 

 leaf, and Fig. 68 a similar portion of the j-jj,. 53 

 under surface of the same leaf, magnified 



200 diameters. In both figures are seen the open stomata between the . 

 Bemi-elllptical cells. The outlines of the other epidermal cells are 



