70 THE LIVING CYCADS 



crown for more than a hundred years; but it often 

 happens that the trunk dies as soon as the bud has 

 exhausted the food materials stored in the ample pith 

 and cortex. 



It is well known that some of the mammoth Sequoias 

 of Cahfornia have reached an age of 3,000 years, perhaps 

 even 4,000 years. The Big Tree of Tule, a cypress near 

 Oaxaca, Mexico, has a trunk fifty feet in diameter and 

 may be even older than the big trees of the Yosemite. 

 But these are very large trees. The cycads are com- 

 paratively insignificant in size, and yet a Dioon with a 

 trunk not more than a foot in diameter and six feet in 

 height may have reached an age of 1,000 years. The 

 plant shown in Fig. i is about 1,000 years old. 



In the big trees the age is determined by counting 

 the annual rings; in the cycads it is estimated by a 

 study of the armor of leaf bases. 



The duration of a crown of leaves varies with the 

 species and with conditions, the leaves persisting one 

 year, two years, three years, or even longer; but after 

 a while the leaflets fall off, the midribs decay, and 

 finally there comes a clean break leaving an inch or 

 more of the leaf stalk, so that the trunk is completely 

 covered by these leaf bases which constitute the ^' armor " 

 (Fig. 22). 



The clean break just referred to is like that which 

 occurs in the case of our familiar shrubs and trees whose 

 leaves fall in the autumn; a peculiar protective tissue 

 develops at the base of the leaf long before the leaf is 

 to drop, so that the wound is healed before it is made. 

 In the cycads this tissue keeps reappearing, scaling off in 

 thin, papery flakes, thus gradually reducing the diameter 



