[baker] 



VESTIGIAL CENTRIPETAL XYLEM 



67 



an interpretation of their origin would explain the parenchymatous 

 shape of the transfusion elements and account for their irregular 

 distribution. It moreover requires no explanation for the lack of 

 transitional elements between centripetal xylem and transfusion tissue. 

 Connection with the secondary wood and the absence of radial arrange- 

 ment both become normal features requiring no especial morphological 

 explanation. Further the theory does not depend for justification 

 on any doubtful hypothesis. It is decidedly less open to objection 

 than the other views advanced and in addition explains the condition 

 foundjin the pine. 



Fig. 19. — Pinus strobus. 

 Cotyledon — long. tang, across cf., — x 450. 

 cf. — Secondary wood; e. — Endodermis; 

 R. — Medullary ray; tt. — Transfusion tissue. 



Conclusions 



A. The evidence from Pinus strobus invalidates the evidence of the origin of 

 transfusion tissue from: 



1. The Fossil Sheaths. 



Owing to the fact that the cotyledonary transfusion tissue does not envelope 

 the entire bundle as in the fossils. 



2. The Secondary Wood. 



In view of the lack of radial arrangement of the transfusion tissue and its 

 varied pericyclic disposition. 



