132 THE BREATHING PORES OF LEAVES. 
double lines in fig. 14 between the ‘‘help pore cells,” as these 
surrounding cells are called, are formed by projections of o 
cell over another, as shown at a of fig. 17, which “a 
from above would show two contours to the same cell ne 
the same plan. 
The development of these stomata is, easily traced in the 
ures. The mother cell, shown in fig. 11, grows less rapid] 
the surrounding epidermis cells, whose Waits therefore stre 
as radii from its four corners. Thin partition walls are t 
across between these radii cutting off from these side ce 
cells as-shown in fig. 12; at @ one of these side cells 
been formed, and two at b. Almost immediately aft 
a pair of end cells are formed in a similar manner; and a 
formation of the four help pore celis, the two pore cells are 
as described in the Gasteria verrucosa. Figs. 15, 16, and 17 
in cross sections the development of these organs, and 
shows the mature state in longitudinal section. The air 
_ do not exist at first, but the unequal growth of the sur 
_ tissues causes tension which splits apart the walls divid | 
cells, and thus forms and enlarges the air spaces ; and in 
manner are formed the openings between the pore cells 
selves. 
The two examples described may serve as types of two 
of stomata, in one of which the pore cells are surroune 
dinary epidermis cells, and in the other by modified cell 
pore cells. Within these two classes are to be found 
differing from each other as variously as the leaves in 
great classes net veined and parallel veined. 
EXPLANATION OF PLATE 3. 
Figs. 1,2,3. . Surface views of epidezmis of Gasteria verrucosa, from 
fAivid 
o the two pore-cells. 
Fig. . posa section of same pN as No. 2., 
‘ig. 
g k dae view of full-grown ‘eat, showing two stomata. 
Hig. n Stomata fully grown seen from beneath, the plane of the di 
Fig. 10. | section of the same. 
