9 8 



THE ANATOMY OF WOODY PLANTS 



passing from the upper part of the figure downward there is a 

 transition from bordered pits arranged in horizontal rows to per- 

 forations of somewhat larger dimensions which are obviously 

 derived from bordered pits by the disappearance of borders and 

 membrane. The truth of this statement can be inferred from 

 the fact that some of the perforations still retain more or less 

 of the original bordered condition, particularly along their lateral 

 margins. Fig. 766 shows another vessel from the root of the same 



FIG. 76. Vessels from root of Liriodcndron Tiilipifera (a and b) ; c, vessel-like 

 elements formed after injury in the non-vascular magnoliaceous genus Drimys. 



genus in which the situation is somewhat different. Here the pits 

 are scalariform, as in the tracheary elements of the Pteridophyta, 

 but with an important distinction which is not always kept in 

 mind in evolutionary speculations. In the ferns and allied forms 

 scalariform elements are a primitive feature of organization of the 

 wood, while in the secondary wood of the angiosperms the scalari- 

 form sculpture of the lateral walls is an exclusive feature of the 

 vessels and is the result of the lateral fusion of horizontal rows of 

 pits and consequently cannot in any way be regarded as a primitive 

 condition of structure as in the lower forms. Proceeding from top 

 to bottom, as in a, we find in this case the same loss of borders and 

 membranes leading to the appearance of perforations of the 

 scalariform type. The structure in question, in fact, exactly 



