MEDULLARY RAYS 93 



by the occurrence of simple teeth, which are sometimes sparingly 

 developed. This feature is intimately associated with the occur- 

 rence of large, simple, and single pits on the lateral walls of the 

 ray cells. From this group P. densiflora may be differentiated by 

 the reticulations in the tracheids of the summer wood. Among 

 the hard pines P. taeda is distinguished by ray tracheids, which are 

 typically sparingly reticulated throughout, but on the other hand 

 P. palustris and P. cubensis, which probably represent the highest 

 types of the genus, are at once separated from all other species 

 by reason of the extent to which reticulations are developed. 



The relations which the tracheids bear to the parenchyma 

 cells in the general composition of the ray also have an impor- 

 tant bearing upon specific differentiations. In the genus Tsuga 

 the tracheids are sometimes interspersed, affording the first 

 instance of a relation which later becomes most prominent in the 

 higher genera, and the same relation is also expressed in Pseudo- 

 tsuga and Larix. In Picea there is a somewhat stronger tendency 

 to an interspersal which is only expressed fully in Pinus. In the 

 soft pines eleven out of thirteen species show, as in the previ- 

 ous genera, that the tracheids, as a rule, are rarely interspersed ; 

 P. aristata forming a partial exception, as shown in a sparing in- 

 terspersal. P. monophylla and P. monticola, on the other hand, 

 show a strong interspersal of the tracheids, and in this respect 

 they approach the hard pines. In the latter group we again find 

 the first four species characterized by a rare interspersal ; but 

 passing on to the more highly developed species, such types as 

 P. clausa, P. palustris, and P. glabra show that the interspersed 

 tracheids are not only numerous but also that they eventually 

 become conspicuously predominant and often constitute the bulk 

 of the ray structure. It is evident, then, that such features pos- 

 sess an obvious value for diagnostic purposes, particularly in the 

 genus Pinus, where the variations are numerous, well defined, 

 and applicable to particular species or groups of species. 



As displayed in tangential section, the medullary ray exhibits 

 two principal forms, each of which presents features of great 

 taxonomic and phylogenetic value. The type of structure which 



