72 THE LIVING CYCADS 



case may be used for illustration. The Mexican Dloon 

 edule forms a new crown every other year, and there are 

 about twenty leaves in a crown, so that the average is 

 ten leaves a year. If there are ten thousand leaf bases 

 the plant is about one thousand years old. A glance at 

 Fig. 2 2 will show that the counting of leaf bases can be 

 done with considerable accuracy. 



This method of estimating the age is very conserva- 

 tive, for seedlings have only one or two leaves the first 

 year, and at ten years they are not likely to produce 

 more than four or five leaves at a time, and crowns are 

 not likely to contain as many as twenty leaves until 

 the plant is at least fifty years old. Besides, when a 

 cone is produced there may be no new leaves in that 

 year. And further, after bearing a cone the plant may 

 be exhausted and may go into a prolonged resting period 

 of three or four years, during which neither cones nor 

 leaves are produced. It is evident that estimates made 

 in this way will be lower than the actual age of the 

 specimens under consideration. 



It would not be safe to apply this method indis- 

 criminately in estimating the age of columnar cycads, 

 for some develop a new crown every year, a few may 

 develop two crowns in a single year, and in some cases 

 the interval between crowns is more than two years. 

 In greenhouses a crown of Dioon edule may keep bright 

 and fresh three times as long as in the field, and pro- 

 longed resting periods are not so likely to occur. 



In Mexico individual plants of Dioon edule were 

 observed in 1904, 1906, 1908, and 19 10, and it was 

 determined that the duration of the crown is two years. 

 Professor Luis Murrillo, who was the botanist of the 



