XXXVIII] COLYMBBTES 493 



into the next, a curving of the elements is frequently evident, 

 and m a few cases it is quite possible to trace a single radial series 

 of tracheids through an angle of 90° running in the same section, 

 first as a transverse and then as a vertical series. One and the 

 same medullary ray also can sometimes be followed, first in trans- 

 verse and then in radial longitudinal section, which later again 

 turns to true transverse. The inference is therefore drawn that 

 there was but a single cambium, which had periodic changes of 





p. pm. b X, y, X, y, x^ y^ x^ y^ Xs" 



Fia. 581. Colymbetes Edwardsi. Diagram of stem in transverse (A) and radial 

 longitudinal (B) section, p, pith; pm, perimeduUary xylem; b, bays of 

 first, vertically running, secondary xylem; x^.x^, etc., zones of horizontally 

 running secondary xylem cut transversely in the radial and radially in the 

 transverse section of the stem; y^, y^, etc., longitudinally running xylem 

 cut transversely in the transverse and longitudinally in the radial section. 

 (After Stopes.) 



direction.' Leaf-traces (fig. 580, It) are large and numerous; 

 they are spirally disposed and pass nearly straight through 

 successive xylem-zones: each trace consists of a small- celled 

 groimd-tissue including stone-cells and patches of tracheids in 

 more or less regular radial rows. Tangential sections of the wood 

 show that the tracheids follow a sinuous course forming loops 

 enclosing numerous medullary rays. 



As the pith and xylem are the only tissues preserved it is on 

 their structure that any speculation as to afiinity must be based. 

 The close arrangement of the leaf- traces (about 1 cm. apart), as 



