FIBRO VASCULAR TISSUES: TRACHEIDS AND FIBERS 33 



point. For example, in the northern species which are placed 

 under the genus Fagus the mechanical elements are of the nature 

 of fiber-tracheids and possess clearly bordered pits. In the 

 antarctic species, on the other hand, the mechanical structures of 

 the wood are libriform fibers. We have thus the remarkable situa- 

 tion that the fibrous elements in certain species of the same genus 

 are derived from tracheids, 

 while in others they are 

 the result of the fusion and 

 modification of storage 

 parenchyma elements in 

 the wood. The situation 

 has only to be stated to 

 make the absurdity of the 

 interpretation obvious. 

 Moreover, while in the 

 woods of the Leguminosae 

 the mechanical elements 



arej for the most part, of FlG 26 ._ M ucilaginous fibers of the black 



the nature of libriform locust (Robinia Psendacacia}. 



fibers, in Bocoa provacensis 



the mechanical elements are represented exceptionally by fiber- 

 tracheids with clearly bordered pits. In certain Rosaceae the 

 transition from fiber-tracheid to libriform fiber may take place 

 within the limits of species (Rhodotypus kerrioides). The inter- 

 pretation of the libriform fiber most in accord at once with the 

 facts of comparative anatomy and with the general trend of 

 evolution, as inferred from the comparison of existing with extinct 

 types, is that it is the product of the extreme modification of the 

 fiber-tracheid, just as the latter in turn takes its origin from the 

 less specialized tracheid. 



An interesting category of mechanical fibrous elements is sup- 

 plied by the mucilaginous fibers found in many woods of widely 

 separated systematic affinities. In these elements the inner portion 

 of the wall has become more or less completely modified into a 

 mucilaginous state which causes it to stain strongly with hema- 

 toxylin. Fibers of this type are found commonly in certain oaks 



