320 PLANT MORPHOLOGY 



and are borne in close spiral arrangement at the apex of the stem. They 

 are rather tough and leathery and vary in length from 5 cm. to 3 m., 

 depending on the species. The venation, often described as parallel, is 

 really dichotomous, as in most ferns (Fig. 2G8A). In Cycas, however, the 

 leaflets have no veins except a prominent midrib. The vernation is 



Fig. 268. Leaves of cycads. A, two leaflets of Zamia skinneri, showing dichotomous 

 venation, one-half natural size; B, young leaves of Cycas circinalis, showing circinate 

 vernation, one-fifth natural size. 



circinate in Cycas and either erect or somewhat circinate in the other 

 genera (Fig. 2685). 



The stem of the Cycadales is like that of the Bennettitales in being an 

 ectophloic siphonostele with a large pith, a thin vascular cylinder, and a 

 thick cortex (Fig. 269). The vascular bundles of the stem are collateral 

 and endarch, but the leaf traces, leaf veins, and bundles of the strobilus 

 axis are mesarch and frequently amphicribral as well. Secondary wood 

 is developed but is commonly small in amount. It consists of tracheids 

 with bordered pits, except in Zamia and Stangeria, where the tracheids 

 are scalariform, like those of ferns. The leaf traces of cycads are peculiar 

 in being double and indirect. This means that, in passing from the stele 



