THE VEGETATIVE STRUCTURES 



83 



In Cycas the leaflets have a midrib but no side veins; 

 Stangeria has both a midrib and numerous reticulated 

 side veins, while in all the rest there is no midrib, but 

 a series of veins parallel with the long axis of the leaf. 

 Taxonomists, having in mind the type of venation seen 

 in grass and lilies, describe these veins as ''parallel"; in 



Fig. 2g.—Dioon edule: portion of a transverse section of a leaflet 



reality they show the forked or ''dichotomous" method 

 of branching which they have retained from their fern 

 ancestors of the Paleozoic age. 



The section of the leaf shows* a structure which 

 affords effective protection against the hot, dry weather 

 of most cycad localities (Fig. 29). The highly cutinized 

 surface is almost impervious to water, and the stomata, 

 or pores, on the under side of the leaf have a form well 



