CYCADALES 



Dioon spinulosum is exceptional in the amount of wood. A stem 

 6 meters in height and 2>2> cm. in diameter had a zone of wood lo cm. 

 in width, with phloem 1.4 cm. in width (fig. 71). What the amount 

 of wood might be in plants with twice that height and diameter has 

 not been determined. The strong medullary rays are a conspicuous 

 feature of the transverse section. 



Fig. 70. — Zamia floridana: transverse section of stem, showing large pith and cortex 

 and scanty zone of wood. Parts of leaf traces are also shown. 



A strange feature in the anatomy of most cycad stems is the ab- 

 sence of growth-rings. Stems of Zamia, Stangeria, and Ceratozamia, 

 which may be more than 50 years old, show no trace of growth-rings. 

 On the other hand, in Dioon spinulosum and in D. edule, growth- 

 rings are so well marked that they can be seen and counted without 

 a lens (fig. 72). The rings are formed by a persistent cambium and 

 they look like the familiar annual rings of dicotyls, but they are not 

 annual or even seasonal. In D. spinulosum they are formed every 

 other year, the stimulus which produces a new crown of leaves mak- 

 ing a growth-ring in the stem. In D. edule even the stimulus which 



