[baker] 



VESTIGIAL CENTRIPETAL XYLEM 



65 



The structure of the cotyledonary leaf varies considerably, as 

 will have been observed, from the other forms. Probably the most 

 marked difference consists in the total absence of transfusion elements 

 below the bast. There is no thin walled tissue present comparable to 

 the vestigial centripetal xylem of the previous leaves. However, in 

 view of its exact centripetal position, the elongated tracheid of Fig. 20 

 may reasonably be considered an element of true centripetal xylem. 



Fig. 17. — Pinus strobus. 

 Cotyledon — long, radial median — x 150. 

 cp. — Centripetal xylem; e. — Endodermis; 

 p. — Parenchyma; tt. — Transfusion tissue. 



The smallness of the amount of this tissue is probably associated with 

 the lack of development of the whole stele. (If a comparison of figures 

 of the steles of the adult and cotyledonary leaves is made (cf. Figs. 

 5 and 15) it will be seen that the former has fully twice as much stelar 

 tissue as the latter.) Moreover, transfusion elements, while continuous 

 with the secondary wood at its flanks, do not show as marked a 

 diminution in length towards the endodermis as in the other types. 



While theoretically a primitive organ some have considered the 

 cotyledon, in view of its peculiar functions, in reality a specialized 

 structure. Consequently the value of any phylogenetic interpretation 

 suggested by the cotyledonary condition might meet with some dis- 

 paragement. However, the position of the cotyledon within the seed 

 is such that the lower portion is turned towards the outside and con- 

 sequently the bast is in contact with the endosperm. In consequence 

 of the absorptive activity of the lower surface one ought to find a well- 

 developed transfusion tissue on the bast side of the bundle. Certainly 



