82 



GYMNOSPERMS 



results in the formation of a new crown of leaves or a cone is not 

 sufi'icient to make a new ring, but when the plant has gone into a 

 prolonged resting period of several years, the stimulus which brings 



Fig. 71. — Dioon spimdosum: transverse section of trunk, showing hirgc pith and cor- 

 tex and prominent medullary rays. The growth-rings show faintly. The zone of wood is 

 the broadest ever described in a cycad. — From Chamberlain, The Living Cycads"' 

 (University of Chicago Press). 



Fig. 72. — Dioon spinidomm: tlic growth-rings are quite clear at tlie rigiit. After 

 Chamberlain. 



it out of its resting period produces a new ring, so that the rings may 

 appear at intervals of 10, 20, or even more years. The rings are due 

 to the alternation of larger and smaller cells, as in dicotyls, but the 



