100 ANATOMY OF THE GYMNOSPERMS 



this genus without difficulty, since a similar feature occurs but 

 rarely elsewhere, and then in such association as to make the 

 differentiation clear. In Juniperus the genus is separable into 

 four well-marked divisions: (i) round to oval or transversely 

 oval, (2) rays broad, the cells oval to round, chiefly round ; 

 (3) chiefly oval ; (4) rays narrow, the cells oblong to oval, chiefly 

 oblong. The broadly oval and thin-walled cells of Sequoia sepa- 

 rate it from associated genera. In Picea the genus may be sub- 

 divided according as the cells are (i) variable, round, oval, or 

 oblong; (2) equal and uniform, oblong or oval. 1 Cupressus is 

 similarly separable into groups. But it is not difficult to separate 

 C. arizonica and C. Goveniana more specifically, by reason of 

 their broad rays and very conspicuously transversely oval cells, 

 from C. pisifera with its round or oval cells and C. thyoides with 

 its narrow, oblong, rarely oval cells. In the genus Pinus atten- 

 tion is at once directed to P. Murrayana by the conspicuously 

 round or transversely oval, very unequal, and variable cells. 



The interspersal of the tracheids often imparts a characteristic 

 appearance to the tangential aspect of the ray, especially in the 

 genus Pinus, and more particularly among the hard pines. In 

 this group the tracheids present very variable forms and sizes. 

 In such types as P. glabra they are small, oval, or round, and 

 wherever they occur they give rise to a marked local contraction. 

 In P. palustris and P. cubensis they are commonly oblong and 

 not infrequently they become several times higher than broad. 

 As they are almost invariably narrower than the associated 

 parenchyma cells, they cause a local contraction which sometimes 

 extends over considerable distances. In P. palustris the predomi- 

 nance of the tracheids is carried so far that the rays are chiefly 

 composed of them, and it then becomes appropriate to apply 

 the term "interspersed" to the few parenchyma cells. In all 

 of the more highly organized rays of the hard pines the appear- 

 ance of the structure is so complex and variable that a proper 



1 The term " equal " here applies to cells of the same ray which are of the same 

 width, " uniform " to the cells of all rays which are pretty constantly of one form, 

 the contrasting terms being " unequal" and "variable" respectively. 



