424 



THE ANATOMY OF WOODY PLANTS 



pores of the tracheary elements in the first-formed region of the 

 annual ring are exclusively radial. The terminal tracheids in con- 

 trast are distinguished by the presence of tangential pitting. It is 

 in the gymnosperms of the middle and later Mesozoic that this 



FIG. 293. Details of organization of wood of Ginkgo. Tangential pitting 

 appears at the end of the annual rings. 



feature is first clearly expressed. It has been suggested by Stras- 

 burger that tangential pitting is a device for supplying water 

 rapidly and abundantly in the new period of growth to the reawak- 

 ening cambium. This seems to be a highly probable explanation 

 of the anatomical conditions present and may be accepted in the 



