i 5 2 STUDIES IN FOSSIL BOTANY 



parenchyma intervenes. The secondary wood consists 

 of regular radial series of tracheides, with medullary 

 rays between them ; the tracheal rows are more numerous 

 than the rays. The secondary tracheides, like the 

 primary, are scalariform (Fig. 61), with the pits on 

 their tangential as well as their radial walls. The rays 

 vary much in height and width ; sometimes a ray 

 consists of a single row of cells ; sometimes it is one 

 cell thick, but many cells high ; while in other cases 

 the middle part of the ray is several cells in thickness. 

 We must remember that these rays are only called 

 " medullary " from analogy with those of other plants ; 

 in Lepidodendron they do not really reach the pith, 

 even where one is present, because the ring of primary 

 wood is quite continuous, so as to shut off the rays 

 completely at their inner ends. The muriform char- 

 acter of the rays, as seen in radial section, is shown in 

 Fig. 61. 



The leaf-traces, or rather their woody portions, ex- 

 tend through the secondary wood, traversing enlarged 

 medullary rays. Quite apart from the leaf-traces, how- 

 ever, the rays generally contain numerous reticulated or 

 spirally thickened elements, which probably served to 

 keep up water-communication in the radial direction 

 throughout the wood ; they would thus be analogous 

 to the tracheides occurring in the medullary rays of 

 many of the Coniferae. 



The phloem underwent comparatively little increase, 

 at least during the earlier stages of cambial activity. 

 In the older stems, where the secondary wood reaches 

 a thickness of half an inch, the phloem is seldom 

 preserved. The actual amount of new vascular tissue 



