380 THE ANATOMY OF WOODY PLANTS 



lattice-work through mucilaginous degeneration. The vessel of the 

 second type is found in the woods of the higher groups and indi- 

 cates a more advanced condition of evolution, other things being 

 equal, than does the vascular type with scalariform terminal 

 perforations. Frequently woods with the porously perforated 

 'type of vessel in their mature structure show the scalariform con- 

 dition in the region of the primary wood, thus providing confirma- 

 tion of the conclusion that the latticed terminal wall of the vessel 

 is phylogenetically older than that in which large pores are present. 

 In many instances vessels in dicotyledonous woods become 

 more or less degenerate and are then easily recognized by their 

 inclined end walls and in any case by a lateral pitting and internal 

 sculpture which clearly distinguishes them from tracheids or fibers. 

 Such vessels are often present in woods in which the fibrous ele- 

 ments are of the nature of libriform, substitute, or septate fibers, 

 and are frequently inappropriately designated as tracheids. True 

 tracheids have always tapering or fusiform ends and are not 

 provided with the lateral pitting and sculpture of vessels. It is 

 important to diagnose degenerate vessels as such, because a failure 

 to make this distinction may result in quite erroneous views as 

 to the relationship of woody dicotyledonous forms. In some 

 cases typical vessels may disappear altogether from the mature 

 structure, either generally or locally. Cases of the general dis- 

 appearance of vascular elements are supplied by certain Cactaceae 

 and Crassulaceae. In the genus Drimys among the Magnoliaceae 

 and in certain genera of the allied Trochodendraceae vessels have 

 also entirely disappeared from the normal wood. In Drimys, 

 interestingly enough, wounds, especially wounds of the root, 

 recall elements which have the lateral sculpture of normal vessels 

 of the Magnoliaceae, without manifesting, however, their charac- 

 teristic scalariform perforations. This defect is easily explained 

 as a result of the comparatively small size of the reversionary 

 elements simulating vessels. Local absence of vessels is frequently 

 found in connection with the development of the large rays in 

 dicotyledonous woods. This is particularly well illustrated in 

 the wood of Alnus and in the root- wood of certain of our northern 

 oaks. In the region of the aggregate ray, as has been indicated 



