﻿EXISTING AND FOSSIL CYCADS COMPARED. 



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also, as each branch bears its own more or less well developed crown of leaves, 

 freely branching trunks are often objects of great beauty. In size the full-grown 

 leaf varies from 10 cm. in length in Zamia pygmaea to more than 3 meters in 

 length in certain species of Cycas and Macrozamia. 



The leaf bases and scale-leaf bases, as seated in the cortex, coverall the surface 

 of the stem as growth proceeds and leave their old bases behind to form the armor. 

 Consequently, as the leaves appear in spiral succession and are closely appressed, 



Fig. 122. — Margin, nervation, and other characters of cycadean fronds or leaves. 



A, Cycas revoluta ; B. Dion spinulosum ; C, Encephalartos horridus ; D, Stangeria paradoxa ; E, Encephalartos villosus (lower portion 

 of frond bearing numerous prickles) ; F, Bowenia spectabilis ; G, Macrozamia heteromera ; H and !, respectively, prefoliation of Cera- 

 tozamia mexicana and Cycas revoluta. Much reduced. — From Engler und Prantl. 



the bases assume in transverse section a lozenge-like shape with all the edges more 

 or less curved either convexly or concavely, and the superior and the inferior angles 

 rounded. Moreover, a double-spiral pattern is formed, made up of a high and a 

 low spiraj, whence in the tangential section through the armor the dexter canton 

 of each leaf-base cut rests higher than the sinister (or vice versa ?). Also the cross- 

 section of each leaf base varies markedly in shape from insertion at the inner 

 border of the armor, where such sections are long and flat, to the periphery, where 

 they are laterally compressed and more rounded, the free petioles of Stangeria and 

 Bowenia being quite round. (See fig. 32, No. 6, showing the petiolar base of 

 Cycas.) 



The fronds are once-pinnate in all the genera but Bowenia, which is bipinnate 

 (see fig. 6) and ancient as indicated by paucity or absence of similar fossil forms. 



