TRANSPIRATION 93 
wall, so that the two cells remain attached to each other by 
their ends (fig. 66). The split constitutes the stoma, and 
Fic, 66.—SuRFAcE VIEW OF PART OF THE UNDER SuRFAcE or A Lear, 
SHOWING THREE STOMATA IN DIFFERENT STAGES OF OPENING AND 
CLOSING 
the two cells-are known as the guard-cells, They are 
commonly of a more or less semilunar form and contain 
some chloroplastids, a point in which they differ from the 
other cells of the epidermis in the higher plants. Their 
walls become thick- 
ened and  cuticu- 
larised, particularly 
those which abut upon 
the slit and upon the 
intercellular space 
(fig. 67); the wall 
which is in contact 
with the other epi- 
dermal cells, however, 
remains thin. When the guard-cells are full of water, 
their form and mode of attachment cause them to become 
Fic. 67.—Srction or Lower EPIDERMIS OF 
A LEAF, SHOWING A Stoma. x 800. 
