BORDERED FITS 



taeda and P. cubensis, as also Larix americana and Tsuga cana- 

 clensis, afford illustrations that, while typically two-seriate, a given 

 section may show a strictly one-seriate arrangement, and this 

 difference also obtains as between contiguous cells. In all such 

 cases examination will show that the variation is directly related 

 to the relative size of the tracheids in such a way that the 

 narrower tracheids, or those arising from 

 a less vigorous growth, are one-seriate. 

 Within the individual tracheid there is 



o 



. 



o 1 o 



o 



. 



o 



s x 



o 



FlG. 9. PlNUS STROBUS. 



^^ Radial section showing the 



FIG. 8. PINUS CUBENSIS. Radial section showing form and disposition of the 



the form and disposition of the bordered pits. bordered pits. X 280 

 X 280 



a strong tendency to a one-seriate arrangement in the central 

 region, while it is two-seriate at the extremities ; and this law 

 holds so true that in those species which are exceptionally two- 

 seriate judgment should be reserved until it is seen that the 

 one-seriate form holds throughout. 



The antithesis of the multiseriate type is found in the one- 

 seriate form. This is typical of 50 per cent of all the species 

 included in the present studies. In such cases the form of the 

 pit is never hexagonal or specially flattened. When the disposi- 

 tion is somewhat compact, as in Firms strobus (fig. 9), the outline 



