268 



THE ANATOMY OF WOODY PLANTS 



tracheary elements, not only on the side which lies toward the 

 secondary wood, but also on that in juxtaposition to the medulla 



or pith. In other 

 words, this calami- 

 tean stem, to which 

 its discoverer has 

 applied the name 

 Protocalamites, 

 somewhat clearly 

 presents xylem of 

 the centrad or cen- 

 tripetal type. 



The secondary 

 wood of calamitean 

 forms was in its 

 early organization 

 largely composed 

 of scalariform ele- 



oei 



FIG. 190. Transverse section through the primary 

 wood of Protocalamites (after Scott). 



ments which in the 

 later development 

 gave place more or 

 less completely to 

 the pitted tracheids. 

 Fig. 191 illustrates 

 the structure of the 

 stem of a calamite 

 in proximity to the 

 primary region. It 

 is clear that the tra- 

 cheids are still very 

 largely scalariform. 

 The pitting of the 

 tracheary elements 

 in Catamites, whether 

 scalariform or rounded, was confined to the radial walls of the ele- 

 ments as in the lower gymnosperms, and was not present on the 



FIG. 191. Longitudinal section of the wood of Calamites 



