X58 THE STRUCTURE OF ECONOMIC PLANTS 



"As indicated in Figure 1x4, C, and Figure 1x5, B, the primary 

 xylem plate is separated into two distinct units by parenchyma. These 

 parenchymatous cells are thin-walled, rectangular in longitudinal 

 section, and polygonal or nearly round in cross section. The radially 



Fig. 115. A, transection of lower hypocotyl, diarch primary xylem strand ; B, transec- 

 tion of upper hypocotyl about two-thirds distance between root and epicotyl ; C, tran- 

 section at slightly higher level in hypocotyl ; large pith ; lateral differentiation of 

 metaxylem ; xylem of bundles of first and second foliage leaves ; D, transection of coty- 

 ledonary plate region ; continuous cylinder of vascular tissue composed of cotyledonary 

 bundles and leaf bundles : w, cambium ; w, endodermis ; wat, metaxylem ; />c/, pericycle; 

 ph, phloem; pi, pith; px, protoxylem ; xy 1, secondary xylem; xji l-i, xylem of first 

 foliage leaf. (After Lyle.) 



arranged phloem groups on either side of the xylem plate have enlarged 

 circumferentially. 



"The diameter of the stele becomes greater at successively higher 

 levels due to an increase in the number of parenchymatous cells at the 

 center of the axis. The units of primary xylem appear to be farther 



