48 Principles of Plant Culture. 
contain about 150,000 stomata to the square inch on its 
lower surface. 
USS es tiili 
Cor each 
ey) Br pA ‘of et) 
Fie. 16. Showing stomata (st.) on leaf of the garden beet. Moderately mag- 
nified. (After Frank and Tscbirch). 
See also Figs. 13, 17 and 20. 
The guard-cells are delicately-balanced valves which are 
extremely sensitive to external influences. They are open 
Fic. 17 Showing stomata (s¢ ) on leaf of Olden- 
burgh apple. Highly magnified. 
in strong light, but usu- 
ally closed in darkness 
and also when the 
leaves are wet. The 
water that escapes 
from leaves (75), and 
the carbonic acid that 
enters them (62) mostly 
pass through the sto- 
mata. The slightly 
raised spots or dots on 
the smooth bark of the young shoots of many woody plants, 
(lenticels (len’-ti-cels)), serve a similar purpose to that of the 
stomata. 
