. STRUCTURE OF STOMATA. 61 
defined from the inner cuticularised layers, and can be stripped 
off as a distinct layer or cuticle (fig. 126, a). 
c. Stomata or Stomates.—These are orifices situated between 
the sides of some of the epidermal cells, and opening into the 
intercellular cavities beneath, so as to allow a free communication 
between the internal tissues and the external air (figs. 132, s, and 
133, 8s). The orifices are surrounded by cells with thinner walls 
and of a different form from those of the epidermis ; they also 
usually contain some chlo- 
rophyll grains. There are Fig. 181. 
generally but two cells sur- 
rounding the orifice, and 
these are commonly of a 
more or less semilunar form 
(fig. 120), so that the whole 
has some faint resemblance 
to the lips and mouth of an 
animal, and hence the name 
of stoma applied to these 
structures, from ord pa, a Hig. yas bp Becton of a portion of the 
° ronda of Marchantia polymorpha. 9, J. 
mouth. These bordering Stoma divided perpendicularly. h, h. Tiers 
cells are called ‘stomatal of cells forming its walls, After Carpenter. 
cells, ‘pore - cells,’ and 
‘guard-cells,’ and have the power of opening or closing the 
orifice which they surround according to circumstances, as will 
be explained hereafter when treating of the functions of stomata 
Fie. 132. Fic. 134. Fie. 135. 
Fie. 133. 
Fig. 132. Vertical section of the epidermis of Lewcadend7on decorum, showing, 
e, e, the epidermal cells, with the stomatal cells, s, with elevated margins, 
m, m.——Fig. 133. Vertical section of the epidermis of the Iris. s. The 
stoma. e,ée. Epidermis. p. Parenchyma beneath the epidermis. J. In- 
tercellular space into which the stoma opens. Fig. 134. Epidermis of 
Rumex acetosa, with rounded stomata, a—-Fig. 135. Square stoma, a, 
of Yucca gloviosa. 
in the part devoted to the Physiology of Plants. Instead of two 
stomatal cells, we sometimes, although bué rarely, find four, or 
even more; thus, in some of the Liverworts, the stomata are 
rounded apertures between the epidermal cells, surrounded by 
