POROXYLEAE 505 



that, according to the observations of MM. Bertrand and 

 Renault, there is no centrifugal primary xylem in the 

 case of Poroxylon. This is a point which might repay 

 further investigation. 



The elements adjoining the spiral protoxylem are 

 scalariform, while the more internal part of the centri- 



Fig. 182. — Poroxylon Edzuardsii. Transverse section of the stem, showing two primary 

 xylem -strands, with adjacent tissues, p, pith ; gc, mucilage-canals ; px, protoxylem ; 

 x, primary centripetal xylem ; x-, secondary xylem ; m.r y medullary rays. X 66. 

 After Uertrand and Renault. 



petal wood consists of pitted elements, exactly as in 

 Lyginodendron or Heterangium. Traced further down- 

 wards in the stem, the structure of the primary wood 

 becomes simplified ; the protoxylem-groups fuse with 

 each other, and ultimately die out. It is a fact well 

 known to anatomists that the spiral elements often 

 disappear in the lower part of the course of a bundle, 

 as is very clearly seen in the Fern Osmunda, the 



