ANATOMY OF THE GYMNOSPERMS 



secondary growth in those cases in which the wall acquires 

 unusual thickness. This is typically the case in Finns cubensis, 

 where in plan (fig. 16) the orifice is extended 



vertically to a length often 



twice the diameter of the 



original pit. In tangential 



section, according to the 



particular direction of the 



plane of section (fig. 17), 



the orifice is either of uni- 

 form width or it enlarges 



FIG. 16. PINUS cu- 

 BENSIS. Radial constantly through the en- FlG I? PlNUS CUBEN . 



sis. Tangential sec- 



section showing 

 deformed bordered 

 pits, x 280 



tire thickness of the later 



tion of bordered pits 

 as in fig. 1 6. X 280 



growth, from within out- 

 wards. That such unusual 

 forms are features of extreme secondary growth of the wall, 

 and that they may be anticipated in all cases where such modi- 

 fications of the walls occur, is a reasonable deduction from the 

 observed facts. 



BORDERED PITS TAXONOMIC AND PHYLOGENETIC 



For taxonomic purposes the bordered pits possess a definite 

 though often limited value. In the genus Cordaites, as also in 

 Araucarioxylon, Araucaria, and Dammara, this is expressed in 

 the hexagonal form together with their very compact, chiefly 

 multiseriate arrangement throughout the entire extent of the 

 tracheids, - - characters which are of generic value and at once 

 serve to separate these genera from all others. The contrast- 

 ing differential feature is then to be found in the pits of the oval 

 or round form, together with their two-seriate or one-seriate dis- 

 position, with a more or less marked tendency to segregation. 

 This is characteristic of the Gingkoales and all the Coniferales, 

 both fossil and recent. 



As a differential character of subgeneric value, the occurrence 

 of bordered pits on the tangential walls of the summer wood of 



