PLANT ORGANS AND ORGANISMS 
173 
number. The mesophyll is differentiated into a central spongy paren- 
chyme containing bundles, and a zone of palisade cells on either 
side facing the epidermises. 
Structure and Development of Stomata. —Stomata are slit-like 
openings in the epidermis of leaves or young green stems surrounded 
o.r. ep. v. p.p. 
ep. p.p. 
Fig. 88 .—Photomicrograph of a transverse section of a bifacial leaf of Eucalyp¬ 
tus globulus showing epidermis (ep.), palisade parenchyma (p.p.), toward both 
surfaces, spongy parenchyma (s.p.), vein (v), and oil reservoir (o.r.) lined with 
secretory epithelium. (Highly magnified.) 
by a pair of cells, called guard cells, whose sides opposite one another 
are concave. They form a communication between the intercellular- 
air-space (respiratory cavity) beneath them and the exterior. The 
slit-like opening taken with the guard cells, constitutes what is known 
as the stomatal apparatus. 
