LIFE HISTORY OF THE MALE FERN 61 
circle. Each of these shows a central xylem mass surrounded by 
an outer phloem mass. Each bundle is enveloped by a per- 
cambium and an endodermis or bundle sheath. 
HisroLocy or A PinnuLe.—When transverse and surface 
sections of the pinnule are studied microscopically, the following 
structural details will be observed (See Fig. 34): 
Fic. 33.—Transverse section of stipe of Dryopteris Filix-mas showing epidermis 
(e); hypodermis (fh); inner cortex (ic); concentric fibrovascular bundles, one of 
which is shown at (+); endodermis (en); pericambium (); xylem (x); and phloem 
(p). (Photomicrograph.) X 50.4. 
1. Upper Epwwermis, of wavy-walled, slightly chlorophylloid, 
flat upper epidermal cells, devoid of stomata (breathing pores), 
but with rather thick cuticle. 
2. MESOPHYLL, a region between the upper and lower 
epidermis consisting largely of chlorenchyma tissue which is 
traversed by vascular bundles. The chlorenchyma contains 
irregular shaped, chlorophylloid cells, with abundant chloro- 
plasts. Intercellular-air-spaces are found between’ various 
cells which are larger in the lower than in the upper region. 
