THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 265 



wood, which is triangular in configuration, with the small elements 

 of the protoxylem at the angles. The primary wood presents no 

 features of special interest as viewed in transverse section beyond 

 the fact that it is protostelic in its organization and consequently 

 lacks a parenchymatous medulla or pith. The secondary wood 

 which surrounds the primary structure is characterized by the 

 radial seriation of its elements and by the presence of medullary 

 rays. There are no 

 other parenchyma- 

 tous structures in the 

 secondary wood ex- 

 cept the rays. The 

 radial parenchyma of 

 the Sphenophyllaceae 

 is peculiar in the fact 

 that its cells, instead 

 of being strictly elon- 

 gated in the radial 

 direction and at right 

 angles to the longer 

 axis of the tracheids, 

 frequently have their 

 greatest dimensions 

 in the vertical plane. 

 This situation leads 

 to the extension of the cells of the rays along the edges upward and 

 downward among the tracheids in a manner simulating true wood 

 parenchyma. This is, however, merely an appearance, for longitu- 

 dinal storage cells of the type ordinarily known as wood parenchyma 

 have not yet been found in any Paleozoic wood of secondary origin. 

 Wood parenchyma, indeed, as has been indicated in an earlier chap- 

 ter, was primitively intimately associated with the phenomenon of 

 annual rings which appeared for the first time in the Mesozoic age. 

 It is clear from the description of the wood of the Sphenophyllaceae 

 supplied in the present connection that it shows, as indeed might 

 be expected, a general resemblance to that of the more ancient 

 representatives of the Lycopodiales. The tracheids were somewhat 



FIG. 188. Transverse section of the stem of 



Sphenophyllum insigne. 



