38 



PLANT RELATIONS. 



Fig. 28. Cells of the epidermis 

 of Maranta, showing the 

 interlocking walls, and a 

 stoma 0) 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 epidermis 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-shaped cells, known as 

 guard- cells, and between them a 

 slit-like opening leads through the 

 epidermis. The whole apparatus 

 is known as a stoma (plural 

 stomata), which really means 



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



lips (see Figs. 28, 29). Sometimes stomata are found only 



on the under side of the leaf, sometimes only 



on the upper side, and sometimes on both 



sides. 



The important 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 why the guard-cells change their 



shape and just what stomata do for leaves. 



They are often called " breathing pores," 



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, therefore, that they hold 



an important relation to green tissue which 



is covered by epidermis. Also, if we examine 



Fig. 2'.). A single 

 stoma from the 

 epidermis of a 

 lily leaf, show- 

 ing the t w o 

 guard-cells full 

 of chlorophyll, 

 and the small 

 slit-like opening 

 between. 



