396 SPHENOPHYLLUM. [CH. 



section are found to consist frequently of a single row of radially 

 disposed elements ; this type of medullary rays occurs in the 

 species Sphenophyllum insigne, in which the tracheae are scalari- 

 form. Three medullary rays, r, are seen on the radial face of 

 the scalariform tracheids in fig. 105, E, which represents a radial 

 section of this species. In other species, e.g. S. plurifoliatum, the 

 medullary rays have a peculiar and characteristic structure ; in 

 a transverse section of the stem they appear as groups of a 

 few parenchymatous cells in the spaces between the truncated 

 angles of the large tracheae (fig. 100). In longitudinal section 

 these medullary- ray elements resemble thick bars stretching 

 radially across the face of the tracheae (fig. 105, B, r): the 

 apparent septa or bars are however thin-walled cells connecting 

 the different groups of medullary-ray cells, as seen in a transverse 

 section. These radial connecting cells are occasionally seen as 

 short rays in transverse sections of stems. 



The cambium and phloem elements are occasionally pre- 

 served in good specimens of older stems; the former consist 

 of tabular flatted thin- walled cells, and the latter in some cases 

 include large sieve-tubes and narrower parenchymatous elements. 



The sections shown in fig. 107, F and F, illustrate the 

 preservation of cambial and phloem tissue. In the transverse 

 section of fig. 107, F, the secondary xylem with the medullary 

 rays, r, is succeeded by a few tabular cambium cells, and external 

 to these there are thin-walled elements of unequal size repre- 

 senting the phloem. In fig. 107, E, the scalariform tracheids 

 are succeeded by narrow thin-Avalled cells, and the larger 

 elements with transverse and oblique septa are no doubt 

 sieve-tubes. 



In the large stem of fig. 105, C, the xylem is succeeded by 

 a band of tissue, a, which is no doubt phloem, and external to 

 this there is a considerable development of periderm (b). The 

 periderm in Sphenophyllum stems had a deep-seated origin, 

 the phellogen or cork-cambium occasionally being formed in 

 the secondary phloem -parenchyma, and in other cases in the 

 pericycle, as in the stems of some living dicotyledons. William- 

 son and Scott ^ describe stems in which a succession of phellogens 

 1 Williamson and Scott (94), p. 926. 



