CONIFERALES (PINACEAE) 



237 



The well-known structure of the leaf is of an extremely xerophytic 

 type, which the section of a pine needle may illustrate (fig. 259). 

 The epidermis consists of elongated, fiber-like cells, with strongly cuti- 

 nized walls, the guard cells being deeply sunken. The rigidity of the 

 leaf is due chiefly to the hypodermal layers or masses of elongated 

 sclerenchymatous cells. The mesophyll is uniform throughout, and 

 it is characterized by peculiar platelike "infoldings" of the wall. 



Fig. 2Sg. — Transverse section of a pine needle, showing epidermis (in which are 

 deeply sunken stomata) underlaid by several layers of heavy-walled cells (scleren- 

 chyma), the mesophyll with characteristic infolded walls and containing resin ducts, 

 the distinct bundle sheath (endodermis) surrounding the vascular region (stele), and 

 the two parallel vascular bundles (xylem directed toward flat or adaxial face of leaf). 



There is a single central bundle region, invested by a very distinct 

 sheath, and traversed by two parallel vascular bundles. The remain- 

 ing tissue within the sheath is the so-called "transfusion tissue," 

 and consists of two kinds of parenchymatous cells : (i) those without 

 protoplasm and pitted, and (2) those with protoplasm and not pitted. 

 The former cells are thought to represent in function an extension of 

 the tracheid system, passing water from the xylem to the mesophyll; 

 and according to Woesdell (45) they have been derived from the 



