THE VEGETATIVE STRUCTURES 71 



of the trunk until the diameter of the lower portion may 

 become less than that of the upper. 



Since the armor, in the columnar forms, persists 

 throughout the life of the plant, it is possible to deter- 

 mine within reasonable limits the number of leaves 



Fig. 22. — Portion of the trunk of Dioon cdulc, showing the armor 

 of persistent leaf bases; also showing three zones marking prolonged 

 resting periods. 



which it has produced. To determine the age it is 

 necessary to count the number of leaf bases and to know 

 the average number of leaves produced in a year. This 

 yearly average can be determined only, by observations 

 extending over a considerable period of time. A simple 



