62 FORMS, ARRANGEMENT, AND NUMBER OF STOMATA. 
three or more tiers of stomatal cells, each tier being itself com- 
posed of four or five cells, the whole forming a kind of funnel 
or chimney (fig. 131). 
Upon making a vertical section through a stomate we usually 
find that the stomatal cells are placed nearly or quite on a level 
with those of the epidermis. In other cases, however, and 
especially when situated upon leaves of a leathery or hardened 
texture, the stomatal cells are below the epidermal ones, while 
in some rare instances, again, they are above them. 
The stomata vary in form and position in different plants, 
and in different parts of the same plant, but they are always the 
same in any particular part of a plant. 
The more common form is the oval 
(figs. 120, s, s, and 121, a, a); but in 
other instances they are round (fig. 
134, a); and in some cases square (fig. 
135, a). They are either placed singly 
upon the epidermis, at regular (jig. 
120) or irregular intervals (fig. 134) ; 
or in clusters, the intervening epider- 
mis having none (jig. 156). The former 
is the more common arrangement. In 
the Banksia we find little cavities be- 
Fig. 136. Epidermis of the leaf Ne Heater ea me be, = er coe: 
AF ok Gucd * Saritraoe, Luce Of the leaves which contain a num- 
dissin cubensis eee ber of hairs (fig. 124, b), and between 
s, with intervening spaces, them, at their base, very small stomata. 
é, €, in which they are ab- ° 
ee 4 The number of stomata also varies 
considerably. The following table will — 
give some idea of their abundance in leaves, and it will be ob- 
served that the number of stomata is usually greatest in those 
leaves where they are entirely absent from their upper surface. 
Fie. 136. 
Stomata in one square inch of surface. 
Upper surface Lower surface 
Mezereon ; : none 4,000" 
Peony . : none 13,790 
Vine. ; , none 13,600 
Olive . : , none 57,600 
Holly: - : none 63,600 
Laurustinus . ; none 90,000 
Cherry-Laurel ' none 90,000 
Tilae: « , : none 160,000 
Hydrangea . : none 160,000 
Mistletoe ; ; 200 200 
Tradescantia . : 2,000 2,000 
House Leek . .», 10,74) 6,000 
Garden Flag . . 11,572 11,572 
