106 ANATOMY OF THE GVMNOSPERMS 



walls, which is pretty fully expressed in A. concolor. A similar 

 recurrence is met with in Pseudotsuga macrocarpa, in Picea 

 polita, and in Pinus Parrayana, and it is also complete in thirteen 

 of the most highly developed species of Pinus, where the walls 

 have suffered extreme degeneration. Within the limits of Picea 

 (i) and the soft pines (5) there are six instances in all of spo- 

 radic and partial survival of the thin and locally thickened wall. 

 The first tendency to thick and strongly pitted walls is mani- 

 fested in five species of Juniperus, and such development is fully 

 expressed in what may be regarded as the three most highly 

 developed species. Thick walls are then fully characteristic of 

 Abies, - - with a partial reversion in A. concolor, - - of Tsuga, 

 Pseudotsuga Douglasii, and Picea, with the exception of P. polita, 

 five species of soft pines, and three species of hard pines. In 

 P. taeda and P. palustris the walls are so degenerate that their 

 structure cannot be satisfactorily determined, but they are pre- 

 sumably thin -walled. 



From these facts it is manifest that the progressive thickening 

 of the terminal walls accords with the general course of devel- 

 opment, and once more making use of the principles already 

 applied to the pits on the lateral walls, we are brought to the 

 natural conclusion that (i) an increase in the thickness of the walls 

 is evidence of a higher type of organization, and (2) that the 

 sporadic recurrence of thin walls with local thickenings represents 

 the persistence of a primitive character. 



Ray tracheids probably constitute one of the most valuable 

 of the structural elements as an indication of development. This 

 has its foundation (i) in the fact, previously shown, that they 

 arise as secondary structures from the parenchyma elements, 

 with which they exhibit interchangeable relations, in direct re- 

 sponse to the requirements of a higher degree of organization, 

 and (2) in their general relation to progressive development. 

 The complete absence of ray tracheids from the Cordaitales and 

 Gingkoales, as also from the Taxaceae and more primitive Coni- 

 ferae, while they are invariable features of the higher Coniferas, 



