38 



PLANT EELATIONS. 



Fig. 28. Cells of the epidermis 

 of M(fraiUa, showing the 

 interlockiiii,' walls, and a 

 stoma (5) with its two guard- 

 cells. 



a network of veins ; (2) a green substance {mesophyll) in 



the meshes of the network ; and (3) over all an epidermis. 

 31. Stomata.' — If a compound microscope is used, some 



very important additional facts may be discovered. The 

 thin, transparent ejiidermis is 

 found to be made up of a layer of 

 cells which fit closely together, 

 sometimes dovetailing with each 

 other. Curious openings in the 

 epidermis will also be discovered, 

 sometimes in very great numbers. 

 Guarding each opening are two 

 crescent-shajied cells, known as 

 guard-CQWs, and between them a 

 slit-like opening leads through the 

 ej)idormis. The whole apparatus 

 is known as a stoma (plural 

 stomuta), which really metms 



"mouth," of which the guard-cells might lie called the 



lips (see Figs. ".'S, ;>!)). Sometimes stomata are found only 



on the under side of the leaf, sometimes only 



on the ujDper side, and sometimes on both 



sides. 



The imi^ortant fact about stomata is that 



the guard-cells can change their shape, and 



so regulate the size of the opening. It is not 



certain just how the guard-cells change their 



shape and just what stomata d(i for leaves. 



They are often called " breathing poi-es," 



but the name is very inappropriate. Stomata 



are not peculiar to the epidermis of foliage 



leaves, for they are found in the epidermis 



of any green part, as stems, young fruit, 



etc. It is evident, tliercfore. that they hold 



an important relation to green tissue wliieh 



is covered by epidermis. Also, if we examine 



Fill. 2'J. A single 

 stoma from the 

 epidermis of a 

 lily loaf, show- 

 ing the two 

 guard-cells full 

 of chlorophyll, 

 and the small 

 slit-like opening 

 between. 



