2 8 THE ANATOMY OF WOODY PLANTS 



and Tertiary had not yet made their appearance in the Paleozoic 

 age. It has been suggested that the function of the tangential 

 pits in the summer wood is the rapid supply of water to the 



FIG. 22. Wood of Paleozoic gymnosperm Cordaites. Explanation in the text 



cambium, or zone of growth, in the reawakening of life in the trunk 

 after the winter period of rest. It is admissible also to interpret 

 the greater mechanical efficiency which marks the summer wood of 

 trees possessing annual rings as of advantage in resisting the 



