Griffin. — Development of New Zealand Conifer Leaves. 51 



material gathered straight from nature I have certainly found 

 undoubted transfusion tracheids and undoubted lignification 

 in the accessory transfusion tissue. 



I should like to add here an opinion concerning the probable 

 origin of transfusion tissue in the species I have investigated. 

 Mr. Worsdell's paper does not leave much doubt as regards 

 the origin of transfusion tissue in those two primitive groups 

 of gymnosperms, the Cycadales and the Gingkoales. In both 

 these groups we see at some period a great development of 

 centripetal xylem. In Cycas it is this wood which does most 

 of the conducting work of the plant in the leaf and petiole, the 

 centrifugal xylem playing quite an inconspicuous part. It is 

 therefore natural here that if any modification takes place in 

 any tracheids for the conduction of water out to the sides, it 

 will be in those of the centripetal xylem. This will be so not 

 only because of their much greater number, but also because 

 the centrifugal wood is probably of very much later develop- 

 ment here, formed after the leaf has been functional for a con- 

 siderable period. In the cotyledons of Gingko the centrifugal 

 wood is again the better developed, and the previous remarks 

 will also apply here. In Mr. Worsdell's figure of the leaf, how- 

 ever, it does not seem very clear as to which elements are cen- 

 trifugal and which centripetal ; the centripetal elements marked 

 are much smaller than those of the centrifugal, and also smaller 

 than an element marked "px," which seems to form a direct 

 transition to the transfusion tracheids at the sides of the 

 centrifugal xylem. It does not, therefore, seem clear in this 

 case why these tracheids should be regarded as formed from 

 the centripetal xylem (vide Trans. Linn. Soc. Lond., Dec, 1897 

 pi. 23). 



When we come to what we consider the more advanced 

 group of gymnosperms — i.e., the Coniferce — the centripetal wood 

 has fallen out of use, its place having been taken by the 

 centrifugal. It seems, therefore, more natural in this case that 

 this wood, which even in the cotyledons has usurped the func- 

 tion of the centripetal in the matter of conduction, should also 

 be the one to become modified for transfusion tracheids. 



When starting on the study of the Podocarpece leaves I fully 

 expected to gain further evidence in support of Mr. Worsdell's 

 theory, and it was only after the development had been traced 

 in several species that I was forced to see that the evidence in 

 the Podocarpece pointed much more strongly in favour of the 

 origin of transfusion tracheids, the greater number at least 

 from centrifugal rather than from centripetal xylem. Mr. 

 Worsdell has said nothing as regards the origin of this tissue 

 in the Podocarpece, having confined himself merely to denoting 



