OF LOWER GREEN'S AX D PLANTS. 33 



tangential sections cut." My o\vn observations on the xyleni 

 lead me to question the preciseness of the histological agreement 

 between the wood of this species and that of living Cycads. 

 I am inclined, indeed, to return to the original description of 

 Carruthers (1870, p. 695), who says, " the minute structure [of 

 the wood] would be described as scalariform, but it is certainly 

 different from the typical form of this found in living ferns, or 

 in the arborescent Ljcopodiacete of the Coal-measures." The 

 wood of this species is peculiar and interesting, and highly 

 suggestive in many respects, and is worthy of the renewed 

 examination in detail which Prof. Seward is undertaking (see 

 p. 46, note). In transverse section the size of the secondary tra- 

 cheids varies, but the largest elements measure about 40 x 38 ^. 



All indications of growth-rings are absent. No secondary 

 or inverted cambiums have been observed in this species, 

 though they occur in many plants of Cycadean affinity. Wood- 

 parenchyma, resin-cells, and resin-canals are not developed in 

 the secondary wood. They appear to be rendered needless by 

 the very numerous and large medullary rays. 



The medullary rays are biseriate or uniseriate. The in- 

 dividual cells of the ray are very large, often as much as twice 

 the tangential diameter of the adjacent tracheids. The rays lie 

 12, a few 3-4, tracheids distant ; as a large proportion of them 

 are only 1 tracheid or 2 tracheids distant, the wood is very 

 loose-textured. Unfortunately, I have no sections which show 

 the rays in good radial section and am not able to determine 

 whether or not their radial walls were pitted. 



The exit of the leaf -trace is an important feature in this 

 group. A single, large, curved bundle passes out from the 

 wood-ring, in which its exit makes a wide gap. 



The phloem is remarkably well preserved and is in con- 

 siderable quantity. It is composed of irregular series of 

 radially arranged secondary elements taking rise from the 

 single cambium, the radial series of phloem, elements con- 

 sequently roughly corresponding to those of the xylem, but 

 their sequence is more irregular. In radial series the phloem 

 is composed of single alternating thick- and thin-walled 

 elements (see text-tig. 10). 



The cortex is composed of large cells, through which are 

 scattered numerous large and irregular gum-passages. Through 



D 



