[baker] vestigial CENTRIPETAL XYLEM 59 



for accepting Jeffrey's view, and believes herself justified in considering 

 this vestigial tracheary tissue true centripetal xylem. This is in 

 agreement with Worsdell's view, though the writer disagrees with his 

 interpretation of the tissue in question as the source of transfusion 

 elements, believing rather that the latter is a tissue sui generis. 



The Polyphylloiis Adult Leaf. Ordinarily, as is well known, the 

 spur shoot of Pinus strobus bears a fascicle consisting of five needles. 

 Thomson (15), however, has recorded several instances of the occur- 

 rence of supernumerary leaves, in acknowledged primitive regions. 

 Moreover, he has shown that these can be induced traumatically, and 

 that morphologically they are more primitive than the ordinary adult 

 leaves, suggesting further that the polyphyllous leaf would possess 

 primitive anatomical features. 



Fig. 10. — Pinus strobus. 

 Adult Polyphyllous — transverse centre. 

 b. — Bast; cf. — Centrifugal xylem; e. — Endodermis; tt. — transfusion 

 tissue; v.cp. — Vestigial centripetal xylem. 



A cross section at about the centre of a leaf taken from a fifteen 

 needled fascicle is shown in Fig. 10. Transfusion elements here form 

 an irregular zone about the entire bundle, and are more abundant than 

 in the normal leaf (c/. Figs. 5 and 10). The inner transfusion cells like 

 those of the normal form are longer than the elements just within the 

 endodermis. A direct lateral connection of transfusion tissue with 

 centrifugal wood is clearly discernible as in the ordinary leaf. Above 

 the protoxylem a large proportion of the pericycle consists of thin 

 walled cells comparable to those found in the corresponding region of 

 the normal adult. When observed in longitudinal section (Fig. 11) 

 their similarity is even more apparent, although in the material 



