THE VEGETATIVE STRUCTURES 73 



State Normal School at Jalapa when I made my first 

 trip, had made a record extending over eleven years of 

 every leaf produced by a specimen of Dioon edule in his 

 garden, and had found that a crown is formed every 

 other year. Estimated by the method just described, 

 this plant, which was less than five feet in height, was 

 970 years old. Another specimen of the same species, 

 about eight inches in height, was known to have been 

 under cultivation for forty years and was presumably 

 a fine specimen when brought in from the field. 



In addition to the leaf bases, which look like leaf 

 scars, many species show a distinct ''ribbing" caused 

 by the alternation of small scale leaves and large foliage 

 leaves, the zone representing the foHage leaves having a 

 larger diameter than that of the scale leaves (Figs. 3 

 and 5). The ribs are conspicuous in the upper part of 

 the trunk, but lower down they become less and less 

 evident, and in very old plants it is practically impossible 

 to identify any trace of the alternation in the lower part 

 of the trunk. The number of ribs shows the number of 

 crowns and thus indicates the age wherever the ribs can 

 be counted and the duration of the crowns is known. 



In some forms, like Dioon edule, there are zones which 

 are not due to the alternation of scale leaves and foliage 

 leaves but to prolonged resting periods. The trunk 

 shown in Fig. i has about a dozen of these zones, some 

 of which can be recognized in the figure. Four such 

 zones are shown in Fig. 22, indicating that there have 

 been four prolonged resting periods during the growth 

 of this portion of the trunk. 



Branching in a cycad is rather rare, but it sometimes 

 occurs both in the subterranean and the columnar types. 



