98 VEGETABLE PHYSIOLOGY 
slightly and splits so as to form an opening between them, 
which does not, however, extend the whole length of the 
Fig. 66.—Surrace Virw oF Part oF THE UNDER SuRFacr oF A LEAF, 
sHowING THREE Stomata IN DirrerRent Sraacrs oF OPENING AND 
CLosINa. 
wall, so that the two cells remain attached to each other by 
their ends (fig. 66). The split constitutes the stoma, and the 
two cells are known 
as the guard - ceils. 
They are commonly 
of a more or less 
semilunar form and 
contain some chloro- 
plastids, a point in 
which they differ from 
Fra. 67._Szotion or Lowrr EripERMIS oF 
a Lear, sHowrna A Stoma. x 300, the other cells of the 
epidermis in the 
higher plants. Their walls become thickened and cuticu- 
larised, particularly those which abut upon the slit and 
upon the intercellular space (fig. 67); the wall which is 
