[baker] vestigial CENTRIPETAL XYLEM 61 



elements intermingled with parenchyma, the former particularly 

 abundant at the flanks of the secondary phloem and at the region of 

 the primary bast. A definite linkage with the cells of the centrifugal 

 wood, just above the cambial region, can be seen, while in Fig, 13 the 

 actual transition between the elements of the two tissues mav be 



Fig. 12. — Pinus slrobus. 

 Primordial, transverse at centre — x 300. 

 b. — Bast; cf. — Centrifugal or secondary wood; e. — Endpdermis; 

 p. — Parenchyma; tt. — transfusion tissue; v.cp. — Vestigial centri- 

 petal xylem. 



observed in the longitudinal tangential plane. Fig. 14 is a tangential 

 section cut through the region between the transfusion elements and 

 the protoxylem, the lateral transfusion cells and the endodermis 

 being shown at the sides of the figure. The thin walled cells in the 

 centre are quite distinct from the transfusion elements, nor could any 

 continuity be found between them. In the two superposed layers of 

 this tissue the end walls are blunt or oblique as the case may be, and 

 the few pits small and faint. Vestigial centripetal xylem is thus 

 present in the primordial leaf and is quite distinct from the surround- 

 ing tracheary elements of the transfusion zone. 



There is thus unmistakable evidence in the three types of leaves 

 of Pinus sir obus studied of the presence of centripetal xylem, not- 

 withstanding Jeffrey's (7) statement to the contrary. Further the 

 elements of centripetal xylem are separated by a distinct line of 

 demarcation from those of the transfusion tissue. There is moreover 

 no indication of transition in pitting between the transfusion cells 

 and the immediately adjacent centripetal tracheids. Thomson (14) 

 in his observation on the attachment of transfusion tissue throughout 

 the Gymnosperms has noted, however, that where there is much 



