no 



MORPHOLOGY OF GYMNOSPERMS 



leaflets of the seedling are conspicuously spinulose, like the adult 

 leaflets of D. spinulosum, but in older plants the margins are per- 

 fectly smooth, the spinulose condition of the seedling of D, edule 

 being a juvenile character indicating an ancestry with spinulose 

 leaves (figs. 84-87). The venation is various; for example, the 

 leaflets of Cycas have only a midrib without any branches; those of 



• Figs. 84-89. — Pinnules of cycads: fig. 84, part of leaf of seedling of Dioon edule; 

 fig. 85, part of adult leaf of same; fig. 86, part of leaflet of seedling of Dioon spinulosum; 

 fig. 87, part of adult leaf of same; fig. 88, part of adult leaf of Zamia floridana; fig. 89, 

 detail of venation of leaf of Zamia floridana; figs. 84-88, one-half natural size; fig. 89, 

 X4. 



Stangeria have a midrib with numerous veinlets ; those of other genera 

 have no midrib, but form more or less parallel veins, some of which 

 run the entire length of the leaflet without branching, while others 

 branch dichotomously and often anastomose (fig. 89). Circinate 

 vernation, characteristic of ferns, appears in the midrib and pinnules 

 of Cycas and in the midrib of Stangeria, Ceratozamia, and Zamia, but 

 otherwise it does not occur. 



The leaflets are tough and leathery, a texture due to thick- walled 



