VESTIGIAL CENTRIPETAL XYLEM 



55 



same feature in regard to the respectively greater length of the inner 

 transfusion elements is shown by Figs. 7, 8 and 9. Fig. 7 represents a 

 portion of a section cut longitudinally in the tangential plane at the 

 fîanks of the secondary wood. Here the suggestion of continuity of the 

 tissues in transverse section is strengthened by the presence of a defi- 



.b. 

 ,.tt. 



.p. 

 .tip. 



P- 



Fig. 6. — Pinus strobus. 

 (Adult, long., radial, lateral and below bundle — x 300.) 

 b. — Bast; e. — Endodermis; p. — Parenchyma; 

 tt. — Transfusion tissue; tt.p. — Pitting. 



nite gradation of elements both in length and calibre, from the cells 

 of the centrifugal xylem to those of the transfusion tissue immediately 

 within the endodermis. The inner transfusion tracheids extending 

 around and above the bundle from this region also are of greater 

 length than the outer tracheary cells. The latter, however, are longer 

 at this point than below the bast. Obviously the greatest length of 

 transfusion elements is attained at the flanks of the centrifugal xylem 

 and especially in those elements in direct contact with it, the elements 

 becoming shorter the farther they are from this region, and attaining 

 their minimum length immediately above and below the bundle. 

 They are shorter, however, below the bast than above the wood. 



This lack of uniformity in length among the transfusion elements 

 of the modern leaf presents a curious anomaly in view of Stopes' (12) 

 interpretation of the origin of transfusion tissue. The elongate 

 character of the elements of the inner transfusion sheath of the fossils, 

 which encircled the entire bundle in Prepinus (Figs. 3 and 4) and when 

 present, enveloped the bast in Cordai tes (Figs. 1 and 2), may be seen 

 from the figures. If transfusion tissue be a derivative of this inner 

 sheath there is every reason to expect the elements below the bundle, 



