THE STEM 



163 



Here the phloem and xylem, instead of constituting a solid mass 

 without any parenchymatous center or pith, are laid down in the 

 form of a hollow cylinder inclosing a central medulla or pith. This 

 type of organization of the central cylinder or stele is technically 

 called siphonostelic, on account of the tubular condition present. 

 Primitively the siphonostele or tubular central cylinder seems to 



FIG. 118. Diagram of a protostelic stem 



have been organized in such a manner that phloem clothed its 

 inner surface as well as its outer one. Further, both inwardly 

 toward the pith and outwardly toward the cortex the tissues of 

 the central cylinder or stele are clearly delimited from those of the 

 fundamental system by a well-marked endodermal layer. The 

 siphonostele may be either concentric or radial in its organiza- 

 tion, precisely as in the protostele, the general situation being 

 identical in the two cases except for the presence of the pith. The 

 essential features of the siphonostele are presented in Fig. 119. 

 Here may be seen cortex and pith marked by a similarity of 



