CYCADALES 93 



Cycadofilicales, and the monosporangiate strobilus separates them 

 from the Bennettitales. The multiciliate sperm, now demonstrated 

 for all the genera except Macrozamia and Encephalartos, and cer- 

 tainly 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 often be- 

 coming quite tall. The Australian Macrozamia Hopei is the tallest, 

 reaching a height of twenty meters; the Mexican Dioon spinulosum 

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

 as tall; while Encephalartos and Ceratozamia form trunks of con- 

 siderable size. The Australian Cycas media, reported as the tallest 

 of the cycads, seldom reaches a height of seven meters. In the tuber- 

 ous 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). All the genera have their branching 

 individuals, but most of the branching is due 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 

 edule, 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, 

 while in Bowenia and Stangeria the leaves break off sharply, so that 

 there is not even the beginning of an armor of leaf bases. 



VASCtJLAR 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" 



