THE STEM 199 
of the deepened primary bundles. The secondary medullary 
rays within the bundles are also called “‘vascular rays.”’ 
As secondary increase progresses, it will be noted that certain 
cells in widely separated regions of the pericycle begin to divide 
rapidly producing groups of thick-walled pericyclic fibers here and 
there within the pericycle but chiefly outside of the protophloem 
of the larger or primary bundles. 
Fic. 129.—Photomicrograph of cross-section of stem of Aristolochia sipho, where 
cambial activity is just beginning. a, Epidermis; 4, collenchyma; c, thin-walled 
parenchyma of the cortex, the innermost cell layer of which is the starch sheath or 
endodermis; d, sclerenchyma ring of the pericycle; e, thin-walled parenchyma of 
the pericycle; f, primary medullary-ray; g, ‘phloem; ’, xylem; 7, interfascicular 
cambium; j, medulla or pith. > 20. (From Stevens.) 
It will be noted that cross sections through the young terminal 
portion of the sunflower stem show an interrupted cylinder of 
separate bundles whereas similar sections cut through the older 
lower portion of the stem show a continuous cylinder of bundles, 
the continuity being due to the laying down by the interfascicular 
cambia of secondary bundles between the primary bundles during 
secondary growth. The stele or central cylinder of the younger 
part of this stem is known as a dictyostele or dissected siphonostele 
whereas that of the older portion is a s¢phonostele (see Fig. 83). 
GROWTH OF PERENNIAL DicoTyL STEM AND Its HisroLocy.— 
A perennial dicotyl stem, in the first year of its growth does not 
