102 



THE ANATOMY OF WOODY PLANTS 



The hypothesis that the scalariform vessel is the forerunner of the 

 porous one is also confirmed by a consideration of the first-formed 

 region of the woody cylinder. In the oak, for example, as well as 

 in many other instances, although vessels with porous terminal 



apertures are characteristic of the 

 mature wood, elements of vascular 

 nature with scalariform perforations 

 are commonly present in the first 

 annual ring, particularly in the vicin- 

 ity of the protoxylem. It is thus evi- 

 dent that the vascular structures in 

 dicotyledonous woods supply a valu- 

 able argument for the general validity 

 of the hypothesis of organic evolution. 

 The lateral walls of the vessels 

 may next receive further considera- 

 tion. It has been indicated in a 

 former paragraph that the side walls 

 of a vessel have their sculptural 

 features largely determined by the 

 nature of the adjacent cells of the 

 wood. Where vessels are in contact 

 with rays, the communicating pits 

 are characterized by a grouping and 

 seriation corresponding to the outlines 

 and direction, whether transverse or 

 longitudinal, of the ray cells. Where 

 the relation is with other vessels or 

 with true tracheids, the pits are 

 numerous and likewise arranged in a 

 somewhat definite manner. If merely 

 mechanical elements abut on the 



vascular walls, pitting is quite absent. Further, if so-called 

 tertiary spirals are present on the inner side of the walls of the 

 vascular elements, these are confined to those vertical regions in 

 contact with other vessels or with tracheids. 



FIG. 79. Vessel of Vaccinium 

 corymbosiim, illustrating the 

 origin of the porous type of 

 perforation from the scalariform. 

 Explanation in the text. 



