BORDERED PITS 



Cordaites . . 

 Araucarioxylon 

 Araucaria 

 Dammara . . 

 Gingko 



I More or less, 



j. often strongly, 



segregated. 



growth is coordinated with a primitive development, while the 

 converse is true of a slow rate of growth which is again con- 

 vertible into terms of maturity. On this basis we may pre- 

 sent the following general outline of sequence in development, 

 as preliminary to further and more detailed discussion of 

 phylogeny : 



. 2-5 seriate, hexagonal pits "1 



. 1-4 seriate, hexagonal pits Compact throughout the 

 . 1-3 seriate, hexagonal pits f tracheid. 



. 1-3 seriate, hexagonal pits J 

 . 1-2 seriate, round or oval pits 1 



Higher Coniferales, 1-2 seriate, round or oval pits 

 Higher Coniferales, i-seriate and pairs, round or oval pits 

 Higher Coniferales, i-seriate, round or oval pits 



Tangential walls. The occurrence of bordered pits in the 

 tangential walls is a well-known and characteristic feature of 

 the Coniferae. In the case of fossil forms, to which Araucari- 

 oxylon offers a partial exception, they cannot be satisfactorily 

 demonstrated because of the peculiar altera- 

 tions of the cell wall, but that they are pres- 

 ent we are permitted to infer from analogy 

 with existing species upon which dependence 

 must be placed for an elucidation of the gen- 

 eral law. The typical position for such pits 

 is upon the tangential walls of the summer 

 wood, where they are seen most satisfactorily 

 in radial section, inasmuch as they are always 

 readily observable when present, and their FIG. n. SEQUOIA GI- 

 most essential features are displayed in a GANTEA. Radial sec- 



. . tion showing bordered 



manner not possible m a tangential section pits in the tangential 



(fi ". II). walls of the summer 



This position obviously results because of 



limitation of the radial walls through radial compression. The 

 pits are therefore always confined to the few outermost tracheids 

 of the last-formed summer wood, and in some cases they are con- 

 fined exclusively to the last tracheid. 



