io8 MORPHOLOGY OF GYMNOSPERMS 



of cycads Matte (40) lays special stress on foliar strands, which he 

 regards as of more phylogenetic importance than the vascular axis of 

 the stem, his unit of structure being the "meriphyte." In this work 

 the leaves and sporophylls of representative species of all the genera 

 were examined, and the seedlings of Cycas, Dioon, and Encepha- 

 lartos. The well-known omega (XI) outline of the foliar arc was 

 found to hold for the whole group, even the complex arrangement 

 of the strands of Bowenia (43) being referable to it. He also dis- 

 covered in the ovulate sporophyll of Ceratozamia (35) three or more 

 accessory vascular strands near the abaxial surface, in addition to the 

 usual transverse row of strands; and mf erred from their inverse 

 orientation (xylem abaxial) that the sporophyll is really a union of 

 two sporophylls. 



THE LEAF 



The crown of large leaves at the apex of the stem gives the cycad 

 its palmlike or fernlike aspect. In Dioon edule, as it grows in the 

 field, a new crown of foliage leaves is produced every other year. 

 When just unfolding, the new crown is easily distinguished from the 

 previous one (fig. 83), but a few months later, after the new crown 

 has fully expanded, the presence of two crowns can scarcely be noticed. 

 In fig. 70 the leaves of the new crown are obliquely erect, while those 

 of the previous crown are horizontal or somewhat reflexed. Below 

 this older crown may be seen some midribs of leaves of a still older 

 crown, the leaflets having fallen off. Below these naked ribs may be 

 found all stages down to the typical crown of persistent leaf bases. 

 It is certain that in greenhouses a crown of Dioon edule may persist 

 much longer than two years. In Cycas revoluta it is said that a crown 

 is formed every other year, but information in regard to the duration 

 of crowns, as they occur in the field, is scanty and uncertain. In seed- 

 lings there is only one leaf, or occasionally two, and for a considerable 

 period the leaves appear singly at irregular intervals, the production 

 of a whole crown of leaves at one time being a feature of the later 

 history of the individual. 



The leaves are once-pinnate in all the genera except Bowenia, in 

 which they are twice-pinnate. They vary in length from 10 cm. in 

 Zamia pygmaea to more than 3 m. in some species of Cycas. The 

 number of leaflets varies greatly even in adult leaves, which in Dioon 



