78 



GYMNOSPERMS 



The trunk in all of the arborescent forms is covered by an armor 

 of leaf bases (fig. 67). The leaf does not fall off as in most deciduous 

 plants, but loses its leaflets, bends down, and decays to a point a few 

 centimeters from the cortex, when an abscission layer appears and 

 cuts the rachis off cleanly, leaving a few centimeters of it to form the 

 armor. Beneath the original abscission layer, embryonic layers ap- 



FiG. 67. — Dioon edule: portion of trunk of an old plant, showing armor of leaf bases. 

 The trunk is smaller below than above. It also shows three zones, marking prolonged 

 dormant periods. — From Chamberlain, The Living Cycads"° (University of Chicago 

 Press) . 



pear in succession and cut off thin membranous sheets so that finally 

 the trunk may have a smaller diameter near the base than it has at 

 a short distance below the crown. 



In most arborescent forms the scaling-ofT of these thin laminae 

 does not progress far enough to obscure the original leaf bases and, 

 consequently, the number of leaves which a plant has borne can be 

 counted even on plants more than a thousand years old. The age in 

 all such forms can be determined with considerable accuracy, if the 



