136 COKTFERALES (RECENT) [CH. 







revoluta. Jeffrey regards the occurrence of Sanio's rims in the 

 cone-axis of Araucaria as a vestigial phenomenon. He failed 

 to find any rims of Sanio in the cone-axes of Cycads and this 

 negative evidence was regarded as favourable to his view that the 

 rims in the Araucarineae are derived from the more fully- developed 

 rims in the Abietineae. Sifton shows that the rims on the tracheids 

 of Cycas revoluta agree closely with those in the xylem of the 

 Araucarian cone-axes and with those in the cone-axis and root of 

 certain Pines. On the assumption that roots and cone-axes are 

 likely to retain ancestral characters, the resemblance of their rims 

 to those found in the Araucarineae supports the view that the 

 Abietineae are descended from ancestois which had rims of Sanio 

 of the Araucarian or Cycadean type. The conclusion is that the 

 shorter rims in the Araucarineae and on the tracheids of the cone- 

 axis and root of the Abietineae represent the primitive form, the 

 broader rims met with in the Abietineae and most other Conifers 

 being later developments. 



The pitting on the walls of medullary-ray cells has in recent 

 years received special attention : in some Conifers the horizontal 

 and tangential walls are strongly pitted (fig. 693, A, G), and this 

 feature is clearly seen in both radial and tangential sections as 

 also, in the case of the horizontal walls, in transverse sections 

 (fig. 693, D, E, F). In most of the Abietineae the pits on the 

 horizontal and tangential walls are a prominent feature while on 

 the other hand in some Abietineae the pitting of these walls is 

 feebly developed: to this type of pitting Gothan 1 has given the 

 name Abietineous pitting. In the great majority of recent 

 genera other than members of the Abietineae the horizontal and 

 tangential walls are smooth (fig. 693, L, 0) ; but there are exceptions. 

 The ray cells in the cone-scales of Agathis are pitted and species 

 of Juniperus 2 , Libocedrus decurrens and Fitzroya also exhibit a 

 form of Abietineous pitting. Gothan points out that in some 

 Junipers and a few other Cupressineae the pits in the tangential 

 walls differ in detail from the typical Abietineous form and that 

 the pits in the horizontal walls are much less distinct than in the 

 Abietineae : there is, however, no very clear distinction between 



1 Gothan (05) p. 43. 



2 Ibid. pp. 43, 45, fig. 7; Stopes (15) p. 63. 



