92 ANATOMY OF THE GYMNOSPERMS 



take the form of teethlike projections into the cavity (fig. 26). 

 In what may be regarded as the most highly developed tracheids 

 the teeth project across the cell cavity until they meet and 

 coalesce, thereby forming a more or less definite reticulation, 

 which gives to the tracheid a very characteristic appearance. 

 As seen in tangential section, such reticulations often appear as 

 narrow bands crossing the cavity from side to side, thus giving 

 the cell a varying aspect. Such dentate and reticulated tracheids 

 are absolutely confined to the second section of the genus Pinus, 

 in which they constitute one of the most characteristic features 

 to the extent of 68.3 per cent of the species. A more detailed 

 analysis of this feature, as applied to the hard pines, is desirable. 

 In P. resinosa and P. Thunbergii the tracheids are simply dentate. 

 In six species, represented by P. Murrayana, the teeth extend into 

 definite reticulations confined to the summer wood ; but in six 

 other species, represented by P. Jeffreyi, such reticulations are 

 sparingly developed throughout the ray. In P. tseda a transitional 

 form appears. Typically this species shows the tracheids to be 

 sparingly reticulated, but occasionally they are strongly reticulated 

 throughout. This brings to mind the further fact that in all 

 species which are sparingly reticulated there is a marked tend- 

 ency to strong reticulation in the summer wood. In the thirteen 

 remaining species the tracheids are uniformly strongly reticu- 

 lated throughout the extent of the ray, and this feature attains 

 its highest expression in P. palustris and P. cubensis. It is there- 

 fore manifest that we have to deal here with a graduated develop- 

 ment of such a nature that the simply dentate tracheid is the 

 most rudimentary, while the strongly reticulated is of the most 

 advanced type of structure. 



The value of the ray tracheid for taxonomic purposes depends 

 upon (i) its occurrence in certain genera, and (2) its structural 

 peculiarities. In the great majority of cases the simple wall of 

 the tracheid affords no basis of specific differentiation, but in the 

 various forms of dentate and reticulated walls of the second sec- 

 tion of Pinus it is of well-defined value in this respect. Pinus 

 resinosa, P. Thunbergii, and P. koraiensis are all characterized 



