CARBONIFEROUS LYCOPODS AND SPHENOPHYLLS. 



67 



bundles, whose individual elements are parallel to each other, run 

 in a sinuous course, the neighbouring bundles bending from and 

 towards each other, and thus form a netted cylinder. The 

 meshes so made form the openings of the primary medullary rays. 

 By exogenous growth from a meristem ring, the bundles increase 

 in size, and form the wedge-shaped bundles separated by the 

 primary medullary rays, so characteristic of Stigmaria. 



The primary medullary rays were continued outwards like 

 spokes of a wheel as additions were made to the outer surface of 

 the bundle through which 

 they passed. The num- 

 ber of the primary rays 

 was therefore fixed by 

 the original number of 

 meshes formed in the 

 young xylem cylinder. 



In tangential section 

 the primary medullar} 7 

 rays are elongate lenti- 

 cular in outline, and are 

 formed of delicate paren- 

 chyma. 



In addition to the pri- 

 mary medullary rays, 

 numerous secondary 

 medullary rays are de- 

 veloped as the xylem 

 cylinder increases in diameter. These are very clearly seen in 

 transverse and tangential sections, and consist generally of a 

 single vertical plate of cells. The number of cells entering into 

 the formation of these secondary medullary rays varies consider- 

 ably. Sometimes they consist of only one cell, or of two, three, 

 or several superimposed cells. 



The ring of bast surrounding the xylem cylinder seems to be 

 very rarely preserved, though Solms-Laubach has " seen tolerably 

 distinct remains of this portion of the rind in a specimen belong- 

 ing to the collection at Strassburs:." x 



Fig. 11. — Stigmaria jicoides, Sternb. sp. 



A, portion of rhizome, natural size ; 



B, rootlet scar — a, vascular cicatrice ; 

 b, circular depression. 



Fossil Botany, p. 274. 



