STRUCTURE OF WOODY STEMS 197 
of cambium. The stems of Dicotyledons and Gymnosperms, 
since they increase in diameter by the addition of new layers 
of xylem or wood on the outside of that previously formed, 
are called exogenous stems. The stems of Monocotyledons are 
called endogenous — a term adopted when botanists had the erro- 
neous notion that monocotyledonous stems grow by the addition 
of new tissues on the inside of the older ones. 
Structure of Woody Stems 
Woody stems, characteristic of the shrubs and trees of 
Dicotyledons and Gymnosperms, are fundamentally the same 
in structure as herbaceous dicotyledonous stems, for the 
GY je ora) wee 
OSU SS 
Fic. 176. — Diagrammatic drawing of a cross section of a young Apple 
twig. e, epidermis; d, cortex; 6, bast fibers; p, phloem; c, cambium; z, 
xylem; a, pith. 
circular arrangement of vascular bundles and presence of 
cambium are likewise their distinctive structural features. 
They, too, are exogenous. Their herbaceous tips, being similar 
in structure to the herbaceous dicotyledonous stems just 
described, need no special attention. (Fig. 176.) Aside from 
