[baker] 



VESTIGIAL CENTRIPETAL XYLEM 



53 



observations in 1904 brought him a step further than Worsdell and 

 led him to conclude "le tissue de transfusion n'est autre chose que le 

 bois centripète." Stopes (12) in her interpretation of the Cordaitean 

 leaf bundle (1903) Figs. 1 and 2, combined Worsdell's view with that 

 of some of his predecessors. She thought the cells of the fossil inner 

 sheath might "represent but little modified elements of it (centripetal 

 xylem)" — but the outer transfusion zone, she considered "phylogen- 

 etically a parenchyma sheath which had acquired bordered pits." 

 Jeffrey (7), however, appears to have associated both these sheaths 

 in the ancestral fossil forms (Cordai tes, Figs. 1 and 2, and Prepinus, 

 Figs. 3 and 4) with the "cryptogamic wood." 



Fig. 4. — Prepinus statenensis. 

 (Long, radial section leaf bundle — Jeffrey 7.) 

 b. — bast. 

 cp. — Centripetal xylem. 

 cf. — Centrifugal xylem. 

 i.S. — Inner, o.s., — outer, transfusion sheath, 

 px. — Protoxylem. 



Obviously a careful study of a single present day Abietinean form 

 in all stages of development should bring important evidence to bear 

 on this much disputed question. Selection from the genus Pinus in 

 particular was considered advisable, inasmuch as the peculiar dis- 

 position of transfusion tissue within the pericycle of this group has 

 led some authors to consider the pine a most primitive type, while 

 others believe it a much modified form. The species strobus was 

 chosen in view of the fact that it afforded such a great variety of 

 easily accessible material. The writer, moreover, was particularly 

 fortunate in having also available polyphyllous brachyblasts pro- 

 duced by the seedling in response to wounding. These were described 

 by Thomson (15) in his work on the "Spur Shoot of the Pines" in 1914, 

 and he suggested that they might yield internal structural evidence 

 of considerable significance from a phylogenetic standpoint. It was 

 possible, therefore, to make sections of cotyledonary, primordial, 



