96 



GYMNOSPERMS 



margin, especially near the tip, while in the adult plant the margins 

 are entire (Tig. 88). 



If the theory of recapitulation holds, it would mean that Dioon 

 spimilosum, with its spinulose leaflets, is the ancestral form and that 

 D. edule shows the spinulose leaflets, in its younger stages, on ac- 

 count of its spinulose ancestry. 



Fig. 88. — Margins of leaflets: A, part of leaf of seedling of Diooti edule, showing 

 spiny leaflets; B, adult leaf with entire margins; C, adult leaf of Diiwti spinulosum with 

 spiny leaflets. — From Chamberlain, The Living Cycads'"' (University of Chicago 

 Press). 



In most, and perhaps all, of the cycads, the leaves of young plants 

 differ from those of the adult, in some the change taking place after 

 the plant is 50 or more years old. Taxonomists have been trapped 

 into identifying two species from leaves taken from a single plant of 

 Enccphalarios altcnstcinii, just as it passes from the spiny to the en- 

 tire leaflet condition. Some of the leaves, at that time, will have 

 spiny leaflets, while in others the leaflets will be perfectly entire, as 



