MEDULLARY RAYS 89 



In Thuya, Cupressus, and Abies the tracheids are strictly mar- 

 ginal in the composite rays, forming the entire structure in rays 

 only one or two elements high. This relation obtains in all the 

 higher Coniferae in the first instance ; but in Larix, Picea, and 

 Finns, where there is a notable increase in numbers, they also 

 become interspersed with the parenchyma cells and eventually 

 predominate over them, a feature which is especially character- 

 istic of the hard pines. Efforts have been made to show that in 

 all such cases the two kinds of elements succeed one another in 

 a definite order from above downward, or the reverse, but our 

 studies have failed to show that this is capable of practical ap- 

 plication to the purposes of classification or even of phylogeny 

 (13, 491). The great fact of importance for our present purpose, 

 however, and one which stands out with much prominence, is that 

 the ray tracheids are not a structural feature of the more primi- 

 tive Coniferales, but only of the higher types, such as Picea and 

 Pinus. Furthermore the primitive position for these structures 

 is in the one- or two-celled rays, or correspondingly in the margins 

 of the composite rays. 



In Thuya and Cupressus the tracheids appear to stand by them- 

 selves, and they exhibit no special relations to the parenchyma 

 elements which would permit of inferences as to their possible 

 origin. In the genus Pinus, on the other hand, where the rela- 

 tions are somewhat complex, evidence does appear of such a 

 nature as to suggest their derivation. In Pinus inops, P. Torrey- 

 ana, P. pungens, P. clausa, P. taeda, P. palustris, and P. cubensis we 

 frequently find thick-walled parenchyma cells and characteris- 

 tic ray tracheids conterminous with one another. This does not 

 mean a simple association, since nearly all such cases, as typ- 

 ically presented by P. palustris, also show a graduated structure 

 of such a nature as to confirm the belief that the one passes into 

 the other by structural gradations. That such is the case can- 

 not be doubted, and if further confirmation were needed, it is 

 afforded by the precisely parallel relations to be met with in the 

 formation of resin cells and resin canals. A further fact of much 

 significance from the standpoint of development is that such 



