194 PHARMACEUTICAL BOTANY 
Endogenous stems are typical of most Monocotyledons and can- 
not increase materially in thickness due to the absence of cam- 
bium. The limited increase in diameter that does take place is 
due to the enlargement of the cells of the primary tissues. Such 
stems show no differentiation into bark and wood regions. 
ee ie t 
0 
spt ie halk 
/ 
Fic. 124.—Sunflower stem. Representative portion of a transverse section 
cut through upper, young part of stem. ep, epidermis bearing trichomes (2); 
hy, hypodermis; co, primary cortex; Pf, pericyclic fibers; p, protophloem of primary 
fibrovascular bundle; c, intrafascicular cambium of bundle; x, protoxylem of bundle; 
m, pith or medulla. All of the tissue between each two bundles is a primary 
medullary ray. Greatly enlarged. 
Hisrotocy or AnnuaL Dicory. STEM.—Excellent stem 
material of an annual dicotyledon for the study of the stems of 
this type is to be found in the sunflower. Transverse sections 
cut through the upper, thin, younger portion of this stem, 
present the following structures: (In both annual and perennial 
dicotyledonous stems, endodermis and pericycle are rarely dis- 
