ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 421 



under a sufficient degree of magnification to show the essential fea- 

 tures of structure. The tracheids are so long that only a small 

 portion of them can be shown in the figure. Their walls are 

 covered with crowded pits arranged in an alternating fashion with 

 no intervening bars of Sanio. The only parenchyma present is 

 radial, and the end of the annual ring is marked by the presence 

 of tracheids which are slightly narrower in radial dimensions but 

 do not otherwise 

 differ from those 

 found in the rest of 

 the annual ring. 



For comparison 

 with the situation 

 revealed by the 

 cordaitean wood 

 from Northern 

 England, a trunk 

 from the Triassic 

 of the southwest 

 region of the 

 United States is 

 shown. In Fig. 290 

 appears a trans- 

 verse section of the 

 wood of a tree from 

 the Triassic forest 



of Arizona. The annual rings are not so distinct in the photo- 

 micrograph as they appear on the weathered end of the actual 

 petrified specimen. It will be clear from the information sup- 

 plied in this case that as far south as Arizona in the Triassic 

 annual rings were more or less clearly marked. A noteworthy 

 variation in the annual temperature in that somewhat remote 

 epoch is thus indicated. This situation presents an interesting 

 contrast to the climatic conditions which prevailed in the region 

 of Prince Edward Island toward the end of the Paleozoic. If the 

 situation be summarized, it is clear that in the later Paleozoic 

 the difference between 46 N. and 54 N. means the presence in the 



FIG. 290. Transverse section of a coniferous wood 

 from the Trias of Arizona. 



