CYCADOFILICALES 5 



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

 contains several concentric vascular strands. There are two kinds 

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

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

 external phloem {ectophloic). The question as to the phylogenetic 



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

 X 35 .—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 sporelings it has been shown that the protostele is the 

 j5rst 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, 



