120 ANATOMY OF THE GYMNOSPERMS 



must have been obliterated in the course of fossilization. \Ye 

 must therefore depend entirely upon such evidence as is afforded 

 by existing species. From this point of view it is obvious that 

 they furnish no evidence as to the origin of either of the three 

 genera in which they occur. It is, on the other hand, possible to 

 determine from other data that both Dammara and Araucaria are 

 much inferior to Abies in point of structural organization and 

 development, and from this we may be permitted to conclude 

 that the resin tracheids of Abies are vestigial forms of elements 

 which were typically developed in Dammara and Araucaria, and 

 possibly characteristic also of their progenitors. If such infer- 

 ences are to be regarded as justifiable, they go far to support 

 the idea of a common origin for all three genera, and they thus 

 lend force to conclusions which lead to the same result, but 

 upon the basis of independent data. 



From a study of the distribution of the resin cells it is appar- 

 ent that they fall into four categories in which the typically 

 segregated cells may be held to represent the most primitive 

 form of disposition. This view is greatly strengthened by the 

 observation that in all such cases the resin cells are rarely if at 

 all accompanied by parenchyma tracheids, while the structure of 

 the cell is farthest removed from that which is found to enter 

 into the composition of resin passages, whence they are also to 

 be regarded as of a primitive character. This view is supported 

 by the observed fact that those genera and species in which such 

 segregations occur are also of a relatively primitive type. With 

 an advance in organization, there is a tendency to the formation 

 of aggregates as expressed in the zonal distribution of Taxodium, 

 Libocedrus, or Sequoia, where we also find the definite forma- 

 tion of groups of cells which later exhibit the initial stages in 

 the formation of a definite canal. But in Sequoia, as also in 

 Abies where similar changes take place, the more complete aggre- 

 gation of the cells is invariably accompanied by structural altera- 

 tions whereby they become greatly shortened and more strongly 

 pitted, while they are always accompanied by parenchyma tra- 

 cheids with which they are interchangeable. In this connection 



