TEANSPIRATION 



93 



wall, so that the two cells remain attached to each other by 

 their ends (fig. 66). The split constitutes the stoma, and 



FIG. 66. SUKFACE VIEW OF PART OF THE UNDER SURFACE OF A LEAF, 

 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 



wifh thp nthpr Pri FIG. 67. SECTION OF LOWER EPIDERMIS OF 

 e l )1 " A LEAF, SHOWING A STOMA. x 300. 



dermal cells, however, 



remains thin. When the guard-cells are full of water, their 



form and mode of attachment cause them to become 



