CYCADOFILICALES 5 



cylinder inclosing pith (medulla); and (3) the poly stele (fig. 5), which 

 contains several concentric vascular strands. There are two kinds 

 of siphonostele: one (fig. 3) with internal as well as external phloem 

 {amphiphloic; often called solefiosiele) , the other (fig. 4) with only 

 external phloem (eciophloic). The question as to the phylogenetic 



Fig. 2. — Gleichenia pubescens: transverse section of stem, showing the protostele; 

 X35. — After Coulter (88). 



connection of these types has given rise to considerable diversity 

 of opinion, but for our purpose the general situation seems clear. 



The protostele is accepted as the most primitive type of stele. In 



the study of sporeHngs it has been shown that the protostele is the 



. first stage in the development of the stele of ferns, whatever may be 



the adult character (13, 31, 36). Among ferns the young protostelic 



stage usually passes into the siphonostelic, but it persists, for example. 



