XI] 



STEM. 



393 



root-like structure, the tracheids having been developed centri- 

 pe tally from the three initial protoxylem groups. This type of 

 structure is typical of roots, but it also occurs in the stems of 

 some recent Vascular Cryptogams. 



As a rule the tissue next the xylem has not been petrified, 

 but in exceptionally well-preserved examples it is seen to 

 consist of a band of thin-walled elements, of which those in 

 contact with the xylem may be spoken of as phloem, and those 

 beyond as the pericycle. Succeeding this band of delicate 

 tissue there is a broader band of thicker- walled and somewhat 

 elongated elements, constituting the cortex. The specimen 

 drawn in fig. 105, J., shows very prominent grooves in the cortex 

 opposite the middle of each bay of the primary wood. It is 

 these grooves that give to the ordinary casts of Sphenophyllum 

 branches the appearance of longitudinal lines traversing each 

 internode. In a longitudinal section of a stem, the cortical 

 tissue (fig. 104, c) is found to be broader in the nodal regions, 



c b a If c 



Fig. 104. Diagrammatic longitudinal section of Sphenophyllum. 



c, outer cortex; b, space next the stele, originally occupied by 

 phloem etc.; a, xylem strand. (After Renault'.) x 7. 



1 Specimen 929 in the Williamson Cabinet is a longitudinal section of the 

 French Sphenophyllum, as described by Renault (70=»). 



