ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 423 



subject further we may point out that the organization of the 

 trunks of trees in succeeding geological epochs indicates a gradual 

 cooling of terrestrial climates which, when it became accentuated, 

 gave rise to a marked variation in annual temperature, recorded 

 ever more clearly in the progressively greater definition of the 

 annual rings in later geological times. 



It is now necessary to correlate the climatic changes more in 

 detail with the 

 internal organiza- 

 tion of the plastic 

 stem organs of 

 vascular plants. 

 It has been shown 

 in Fig. 289, repre- 

 senting the wood 

 of the cordaitean 

 Mesoxylon of Car- 

 boniferous age 

 from the north of 

 England, that the 

 only differentiation 

 which marked 

 annual increments 

 of growth in this 

 early age was a 

 slight narrowing 



in radial diameter of the tracheids formed toward the end of the 

 period of growth. No other structural feature related to the 

 phenomenon of annual rings has been recorded as yet from either 

 the late Paleozoic or from the early Mesozoic (Triassic). In the 

 Jurassic, however, and practically universally from this epoch 

 onward, a marked modification in the organization of the annual 

 rings presents itself. This is well shown in the structure of the 

 wood of Ginkgo (Fig. 293), a survivor of a stock which attained its 

 zenith of development in the Mesozoic age. Clearly the tracheids 

 which mark the termination of the annual increment differ from 

 those previously formed in the character of their pitting. The 



FIG. 292. Transverse section of the wood of Agathis 

 australis from New Zealand. 



