68 THE LIVING CYCADS 



from the seed plants, although the series, like that often 

 existing between two species within a genus, is one» with 

 easily recognizable extremes, but with intergrading 

 forms which can be classified only arbitrarily. 



During the Mesozoic the aspect of the group became 

 more like that of the living cycads, so that they were 

 no longer mistaken for ferns, but were mistaken- for 

 true cycads, and botanists characterized the Mesozoic 

 as the "Age of Cycads." Technically most of the 

 Mesozoic cycads, now represented only by fossils, are 

 called the Bennettitales (Fig. 78). 



In general appearance the living cycads would not 

 be easily distinguished from some of their Mesozoic 

 ancestors, for both have the armored trunk surmounted 

 by a crown of leaves. The distinctions are found in the 

 reproductive structures and in details of internal struc- 

 ture. There is little difficulty in applying names, since 

 all the Bennettitales are extinct, and all the living cycads 

 are called Cycadales, so that extinct members of the 

 Cycadales are the only ones which could cause confusion. 

 While there must be such extinct members in the 

 Mesozoic, very little is known about them. 



In the living cycads there are two types of stem, 

 one subterranean and tuberous, the other aerial and 

 columnar. 



The tuberous type is represented in both hemispheres; 

 in the Western by Zamia, and in the Eastern by Bowenia, 

 Stangeria, and by some species of Macrozamia and 

 Encephalartos. This stem varies in size from an inch 

 in diameter and a few inches in length in Zamia pygmaea, 

 to a foot in diameter and two feet in length in Macro- 

 zamia. In some cases the bud and leaf bases are above 



