406 



STUDIES IN FOSSIL BOTANY 



exactly in structure with one of the primary strands of 

 xylem in Lyginodendron (compare Fig. 156 with Fig. 

 131). The smallest elements of the cluster are internal, 

 lying at a short distance from its outer limit (Fig. 156, 

 px) ; these smallest elements are shown by longitudinal 



sections to be tracheides, 

 with a lax spiral thicken- 

 ing ; they thus constitute 

 the protoxylem of the 

 bundle (Fig. 1 5 7,px). They 

 are succeeded towards the 

 exterior by more densely 

 spiral or reticulate trach- 

 eides, of somewhat larger 

 size (Figs. 156 and 157, 

 x 1 ). The protoxylem is 

 accompanied by a little 

 parenchyma, and the rest of 

 the primary strand, includ- 

 ing all its lateral and interior 

 part, is formed of large 

 tracheides, with multiseriate 



Fig. 157. — Corresponding longitudinal sec- 

 tion, showing the structure of the 

 tracheides. .rV parenchyma at the bordered pits, aS in LyginO 

 commencement of a medullary ray. ■ yo 



Other lettering as in Fig. 156. X 135. 

 Phil. Trans., W. and S. Will. Coll. 

 1266. 



dendron (Figs. 156 and 157, 

 x). Similar pitted elements 

 extend throughout the whole interior of the stele, forming 

 the mass of primary wood. The central wood, as dis- 

 tinguished from the peripheral xylem-strands, may be 

 termed the metaxylem. From the nature of the case, 

 the primary strands are not obviously distinct, as in the 

 former genus, but they are characterised at once by 

 their mesarch structure, and by the fact that they alone 



