38 
PLANT STUDIES 
FIG. 28. Cells of the epidermis 
of Maranta, showing the 
interlocking walls, and a 
stoma (s) 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 how 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. 29. A single 
stoma from the 
epidermis of a 
lily leaf, show- 
ing the two 
guard-cells full 
of chlorophyll, 
and the small 
slit-like opening 
between. 
