SPHENOPH YLLEAE. 



347 



each wing are two weak initial strands, lying close together and composed 

 of spiral and annular cells with threads that may be unwound, and as there 

 is usually a small gap in the tissue between them, they project into it and 

 form two teeth at the angle of the xylem-strand. The entire xylem-strand 

 is usually uniform, though in the young state its tracheal elements would 

 appear to be accompanied with a parenchyma, which may be supposed to 

 be destroyed at a later period, as the tracheides increase in diameter though 



FIG. 48. i. Longitudinal section of the stem of Sphenophyllum quadrifidum. 2. Transverse section through the 

 same, surrounded by the transverse sections of the leaves, each of which is four-nerved ; in the centre the three-winged 

 primary xylem-strand, enclosed in a single layer of secondary wood. 3. Longitudinal section of the stem through the 

 secondary wood, somewhat oblique, so that it is tangential on the lower side and almost radial on the upper. 

 4. Transverse section of the stem of Sphenophyllum Stephanense, B. Ren., showing the leaf-strands and their attachment 

 to the initial groups of the three-winged woody body ; four layers of secondary wood already formed. From Zittel's 

 text- book. 



probably not in number 1 . Renault z has found a macerated silicified stem 

 in which the xylem-strand, by destruction of the more central tissue, is 

 broken up into three curved rows of vessels with their convexity turned 

 towards each other, and with their six free extremities formed of six initial 

 bundles. He supposes therefore that the whole body was originally com- 

 posed of three separate diarch strands. Van Tieghem 3 indeed thinks that 

 it was developed from six separate monarch strands united together in pairs, 

 and having their initial bundles on the outer side. Either view is possible 



1 Renault (j2~), vol. iv, Introd. p. 7. 

 Tieghem (3), p. 173. 



3 Renault (ty, vol. ii, Introd. t. A, f. 2. 



