ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 427 



the Jurassic and Cretaceous, storage elements are already found 

 distributed throughout the annual ring. Among the living conif- 

 erous subtribes all but the Abietineae have or formerly possessed 

 diffuse parenchyma or storage tissue scattered throughout the 

 annual ring. This condition, for example, actually holds for the 

 Taxodineae, Cupressineae, and Podocarpineae, and there is good 

 evidence either from fossil forms or conservative organs or from 

 both together that the Taxineae and Araucariineae formerly 

 possessed abundant, diffuse storage parenchyma. In the Abie- 

 tineae a very interesting condition exists which is quite in 

 harmony with the ancestral position assigned to them in an earlier 

 chapter. In the group Pineae parenchymatous elements are 

 absent in the ancient Pinus and Prepinus; while in Picea, Larix, 

 and Pseudotsuga they occur in an exclusively terminal position 

 and present, particularly in Picea (notably in the root of the various 

 species), all transitionary stages of derivation from septate tra- 

 cheids. It may be assumed that in the Pineae we have the most 

 primitive conditions found in the conifers as regards the presence, 

 position, and mode of origin of parenchymatous elements. In the 

 Abieteae, comprising Abies, Tsuga, Cedrus, and Pseudolarix, the 

 longitudinal storage parenchyma is characteristically terminal, 

 but no longer shows normal evidence of derivation from tracheary 

 elements. In wood formed after injury, however, transitions 

 from tracheids to parenchyma cells can readily be observed. The 

 genera Abies and Tsuga are of particular interest among the 

 Abieteae on account of the fact that some of their species show 

 a transition toward the diffuse condition of distribution of wood 

 parenchyma. In A . webbiana and A . cephalonica storage elements 

 scattered throughout the annual ring have been observed, and a 

 similar statement holds for the mature wood of T. mertensiana 

 and the branches of T. canadensis. In the remaining subtribes 

 of conifers, as shown above, the parenchyma is or has been diffuse 

 in its mode of occurrence. Further, it presents no normal tran- 

 sitions from tracheary elements, although such conditions can 

 frequently be observed in injured material. 



The conifers have been chosen to illustrate the correlation 

 between the organization of wood and climatic conditions," 

 because their great geological age and the abundance of their 



