312 



CALAMITES. 



[CH. 



xylem groups is shown in fig. 74, A. External to the carinal 

 canal, the xylem tracheids are disposed in regular series and 



^TZ 



A \ B 



Transverse section of part of a Calamite stem. [Calamites 



(Arthropitys) communis (Binney).] 

 s, Stigmariau appendage, x, xylem. From a specimen in the 



Binney Collection, Cambridge, x 50. 

 B. Transverse section of a stem. 



ft, hypodermal tissue ; c, inner cortex. From a specimen in the 



Williamson Collection (no. 62). x35. 



associated with numerous narrow secondary medullary rays. 

 The width of the xylem wedge increases gradually as we pass 

 outwards, this is due to the formation of interfascicular xylem, 

 which in the more peripheral portion of the stem extends 

 across the primary medullary rays. The few primary medullary- 

 ray cells shown in the drawing illustrate the characteristic 

 tangentially elongated form and large size of the parenchy- 

 matous elements. Williamson and Scott have pointed out that 

 the tangentially elongated form of the medullary-ray cells is the 



