76 THE LIVING CYCADS 



but after a thorough drying it becomes extremely Hght, 

 even lighter than the dryest white pine. 



Growth rings, which are such a conspicuous feature 

 of woody plants, were supposed to be entirely lacking 

 in the cycads, until they were found in Dioon. In Dioon 

 spinulosum these rings are formed whenever a new 

 crown of leaves is produced; but in Dioon edule a ring 

 marks the alternation of growing periods and prolonged 

 resting periods, the mere alternation of rainy and dry 

 seasons not being sufficient to cause any noticeable 

 inequality of growth. If crowns of leaves are formed 

 every other year, the number of growth rings in 

 D. spinulosum would indicate half the age of the plant; 

 in D. edule they would give no clue to the age of the 

 specimen, since the number of rings does not correspond 

 to the number of crowns or the number of cones, but 

 to the number of prolonged resting periods, which 

 probably come at irregular intervals. A plant of 

 D. edule about one hundred years old showed twenty 

 growth rings. 



In most cycads the growth of the woody zone is 

 continuous, but in Cycas, Macrozamia, and Encephalartos 

 the growth of the woody zone ceases, and after a time 

 a new zone appears in the cortex, and this phenomenon 

 may be repeated, so that the result looks like a few 

 enormously large growth rings. These successive zones 

 of wood . doubtless mark the alternation of growing 

 periods and periods of prolonged rest. 



A puzzling feature of some cycad stems is the ''cone 

 dome." Inside the main zone of wood some genera, 

 like Dioon, Ceratozamia, and Stangeria, show a small 

 zone which, lower down, becomes connected with the 



