362 STUDIES IN FOSSIL BOTANY 



next outer one (l-t?) is separated from the former by 

 i| of the circumference, the next outer one again {l.t. ) 

 by another ^, and so on through the whole circuit. 

 This arrangement of the leaf-traces points clearly to 

 a -| arrangement of the leaves themselves, which is 

 confirmed by direct evidence, when we have pieces of 

 the stem long enough to show the bases of two or more 

 leaves. The phyllotaxis, then, was a spiral one, with 

 a divergence of |, at least in the larger stems. Of 

 course phyllotaxis is a variable character ; in the very 

 small stems of Lyginodendron there appears to have 

 been a ^ arrangement, and other divergences may very 

 probably be met with. 



As the leaf-traces of five successive leaves appear in 

 the same transverse section, it follows that each leaf- 

 trace must have extended through a length equal to 

 five internodes, between leaving the wood and entering 

 the petiole. As the internodes were often about an 

 inch long, it is evident that the outward passage of the 

 bundles was a very gradual one. The strands of wood 

 around the pith are, as we have seen, continuous with 

 the leaf-traces, and, in fact, form the lower part of the 

 same system. Their number is less constant than that 

 of the more external leaf-traces ; sometimes it is five, 

 sometimes more. Where there are five xylem-strands 

 only around the pith, they are alternate with the five 

 leaf- traces in the pericycle. Where there are more 

 than five, the excess is due to the fact that some of 

 them are double ; their alternation with the outer traces 

 is still maintained (Fig. 129). Each of the bundles 

 surrounding the pith is, in fact, a sympodium, composed 

 of the united lower ends of successive adjacent leaf- 



