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 



ep. v. p.p. 



ep. p.p. 



PIG. 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. 



