[b\ker] 



VESTIGIAL CENTRIPETAL XYLEM 



63 



appreciate the fact that "tnie centripetal xylem is opposite the proto- 

 xylem." (Thomson 14.) Thus his homology can only apply to those 

 elements immediately above the protoxylem. 



Fig. 14. — Pinus strohus. 



Primordial long. tang, above protoxylem — x 320. 



e. — Endodermis; tt. — transfusion tissue; ttp. — Pitting on tt; 



v.cp. — Vestigial centripetal xylem, v.cp.p. — Pitting on v.cp. 



The Cotyledonary Leaf. The cotyledon is regarded as one of the 

 most primitive types of leaf. A section cut transversely at about the 

 centre (Fig. 15) shows within the irregular endodermis (bounding the 

 figure) a pericycle of, for the most part, two largely parenchymatous 

 layers. There is at the left an obvious connection of transfusion tissue 

 with the secondary wood. This is not so manifest at the right owing to 

 the considerable admixture of parenchyma at this point. The tendency 

 of the transfusion tissue to dispose itself in the arc of a circle just 

 within the endodermis above the xylem portion of the bundle is also 

 demonstrated by the figure. No tracheary elements are found on the 

 phloem side of the bundle. Fig. 16 is a drawing of a radial section cut 

 in the longitudinal median plane. Fig. 17 shows at a lower magnifi- 

 cation one of the very long centripetal xylem elements. In both 

 figures, as usual, the lower surface of the leaf is towards the right. 

 Immediately at the left of the protoxylem a single row of elongated 

 parenchyma cells occurs, followed by a long narrow tracheary element. 

 To the left of this again, and between it and the endodermis, there is 

 found a row of elements suggestive of elongated transfusion tracheids. 

 A tangential longitudinal section above the protoxylem is partly 

 represented by Fig. 18. Here within the endodermis there is a tier of 



Sec. V, Sig. 5 



