No. 449.] NORTH AMERICAN CONIFERALE: 



349 



ciated, but this rule is subject tc 

 always distinguished by a broad- 

 ening of the central tract by 

 two to several times the original 

 dimensions, thereby becoming 

 more or less multiseriate. These 

 variations depend upon the na- 

 ture of the included structure 

 which exhibits modifications di- 

 rectly related to progressive de- 

 velopment of the genus. Such 

 broadening arises abruptly in 

 Pseudotsuga, Larix and Picea, 

 so that the terminals above and 

 below consist of a single series of 

 cells with the general structure 

 of the uniseriate ray (Fig. 28). 

 In Pinus the broadening is less 

 abrupt, diminishing in both direc- 

 tions somewhat gradually, thus 

 giving rise to a region of lenti- 

 cular form, which occupies up- 

 wards of half the height of the 

 ray, or in some cases constitutes 

 the entire structure. From this 

 it follows that in such types as 

 P.pahistris (Fig. 29^), the ter- 

 mmals which are often prolonged 

 to great length, may be linear 

 and uniseriate, while in P. clausa 

 the whole ray is lenticular in 

 outline and the terminals consist 

 of only one or two limiting tra- 

 cheids (Fig. 30). Within the 

 region of the central tract the 

 cells are all thick-walled in Pseu- 

 dotsuga, Larix and Picea, but in 

 Pinus the cells are generally thin- 



several exceptions. They are 



