OF XYLEM 



H. B. Sift on 



ONE 



The detailed investigation of certain anatomical features of the 

 Cycads has been undertaken in the hope of throwing light on the 

 origin of the more specialized structures occurring in the higher 

 Gymnosperms. In recent years considerable work has been 



determining 



Conifers. These 



prominence as 



evidences of the inter-relationships of the various groups. A lack 

 of knowledge of the ancestry of the structures themselves, however, 

 has minimized their value as criteria in phylogenetic investigations. 

 This knowledge can be supplied only by a study of primitive forms. 



Pitting 



The shape and arrangement of bordered pits in the woody 

 tissue have long been regarded as valuable phylogenetic data. It 

 is largely owing to these features that the Araucarians have been 

 supposed to be closely related to Cordaitean forms, and many 

 botanists still hold this view, notwithstanding the arguments 

 advanced by Jeffrey (6) and Seward (9) in favor of different 

 lines of ancestry for the family. In 1907 Gothan (4) worked out 

 a phylogenetic line of development of bordered pitting, considering 

 the most primitive type to be hexagonal and crowded over the 

 whole tracheid wall. According to his theory, the pitting next 

 became eliminated from the tangential walls, but still covered the 

 radial as before. A further elimination resulted first in small 

 isolated groups of pits, then in the uniseriate flattened condition, 

 and finally in the scattered arrangement, where the pits occur 

 singly on the tracheid wall. This series of eliminations took place 

 on the middle part of the wall of the tracheid, the crowded arrange- 

 ment being retained on the ends to facilitate vertical movement 

 of the sap. Gothan found his types of arrangement combined 



form 



425] 



[Botanical Gazette, vol. 70 



