326 



THE ANATOMY OF WOODY PLANTS 



they are clearly antecedent to the mass of other living coniferous 

 subtribes. 



The mature wood of Pinus, illustrated in Fig. 234, is char- 

 acterized in transverse section by the presence of resin canals and 

 by rays of complex organization. The longitudinal structure of 

 the wood, as is shown in Fig. 235, is characterized by the presence 



of bars of Sanio. The 

 rays are of complicated 

 structure, even when uni- 

 seriate, and are composed 

 of central storage cells 

 and marginal elements 

 resembling tracheids. If 

 a primitive region such as 

 that provided by the 

 wood of the cone axis or 

 root be investigated, it 

 becomes clear that the 

 bars of Sanio are not an 

 ancestral feature of 

 organization of the wood, 



since they are absent in 

 FIG. 234. Longitudinal view of the wood in . . ... 



Pinus resinosa. the lnner ^S 1011 f the 



wood of the cone and are 



also often lacking in the tracheary elements of the root, especially 

 in proximity to the primary wood. This situation is portrayed 

 for the cone of the Italian nut pine (Pinus pinea) in Fig. 236. 

 It is obvious that the tracheids retain for some time the spiral 

 markings of the primitive region. The walls of the tracheary 

 elements show not the slightest indication of the presence of bars 

 of Sanio until a region remote from the pith has been reached. 

 The rays also are without the marginal tracheids which manifest 

 themselves at an early stage in the organization of the wood of 

 the vegetative branches. Clearly, so far as the structure of the 

 wood in the reproductive axis is concerned, the Abietineae as 

 represented by Pinus are derived from ancestors possessing the 

 structure of the wood of the Cordaitales. Another interesting 



