CYCADALES 93 



Cycadofilicales, and the monosporangiate strobilus scj)arates them 

 from the Bennettitales. The muhicih'ate sperm, now demonstrated 

 for all the genera except Macrozamia, Bowenia, and Encephalartos, 

 and certainly present in these three also, is a character shared by 

 Ginkgo, and doubtless by all of the extinct orders. 



I. The vegetative organs 



The stems are columnar or tuberous, the former kind prevailing 

 and often becoming quite tall. The Australian Cycas media reaches 

 a height of more than twenty meters; the Mexican Dioon spinulosum 

 (fig. 71) becomes twelve meters high; the Cuban Microcycas is nearly 

 as tall; while Encephalartos, Ceratozamia, and some species of Macro- 

 zamia form trunks of considerable size. In the tuberous forms the 

 stem is either entirely subterranean or appears more or less above 

 the surface. All stems are typically unbranched, but branched 

 individuals are not rare in cultivation and are readily found in 

 the field (fig. 72). Doubtless all the genera have their branching 

 individuals, but most of the branching is due probably to injuries 

 or to the germination of seeds in the nest formed by the crown of 

 leaves. 



A striking feature of the cycad trunk, especially in the columnar 

 forms, is the investing armor of leaf bases, recalling the large persist- 

 ent leaf bases of the marattiaceous ferns. In some forms, as in Dioon 

 ediile, the armor is so persistent that even in an old plant the number 

 of leaves which it has produced can be determined with reasonable 

 accuracy. From the number of leaf bases, the average number of 

 leaves in a crown, and the duration of the crown, the age of a plant 

 can be estimated. The plant shown in fig. 70, with a trunk less than 

 two meters high, is probably about 1,000 years old. In the smaller 

 tuberous forms and in some species of Cycas, the armor is not so per- 

 sistent, and may be visible for only a short distance below the crown, 

 the lower portion of the stem becoming quite smooth and free from 

 leaf bases. 



VASCULAR ANATOMY 



A transverse section of the stem of a cycad (fig. 73) shows a large 

 pith, a relatively thin vascular cylinder of collateral endarch bundles, 

 and a very thick cortex containing the numerous conspicuous " girdles" 



