350 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



vascular cylinder thin, and the pith large. Bundles are collateral 

 and endarch in the stem, but often mesarch and even concentric in 

 the leaf traces and strobilar axes. The leaf traces girdle the stem, 

 entering leaves on the opposite side from which they separate from 

 the vascular cylinder. Wieland has unquestionably shown that the re- 

 lationships between the existing and the two extinct orders are closer 

 than was formerly supposed. 



The existing cycads constitute a rather compact group of nine 

 genera and about 110 species indigenous in the warmer regions of 

 both hemispheres — Cycas with some 10 oriental species and Zamia 

 with about 30 occidental species being the dominant genera. Be- 

 sides Zamia the genera Microcycas, Dion, and Ceratozamia are 

 American; Encephalartos and Stangeria are African, while Cycas, 

 Macrozamia, and Bowenia are Asiatic or Australian. In individual 

 abundance they do not play a leading role in any forest assemblage, 

 although the small Stangeria paradoxa forms thickets in southeastern 

 Africa and Macrozamia spiralis forms a scrub over considerable 

 areas in eastern Australia. For trunk-forming plants they are in- 

 conspicuous and vary from the dwarf Zamias with underground 

 stems or epiphytic forms to some of the species of Cycas, which 

 reach a height of 20 meters and live to a great age. 



The stem consists of pith, wood, cambium, and phloem, enveloped 

 in a thick cortex ending in periderm and giving rise to a thin bark, 

 but supporting an investure or armor of old leaf bases, persistent 

 or not, according as the growth of periderm affects its excision. 

 The pith occupies one-third the diameter of the stem. It is ex- 

 tremely parenchymatous and frequently affords commercial starch. 

 Mucilage canals traverse the pith, cortex, and rays. The pith is 

 traversed by gum canals and by anomalous cauline vascular bundles 

 in Encephalartos and Macrozamia and by peduncular bundles in 

 Dion, Stangeria, Ceratozamia, Zamia, etc. The wood falls into two 

 types. In Zamia, Dion, Stangeria, Ceratozamia, and Microcycas (?) 

 it is what is known as monoxylic, i. e., there is a thin and more or 

 less open system of collateral bundles and no persistent cambium or 

 marked secondary thickening. Contrasted with this, Cycas, Macro- 

 zamia. Encephalartos, and Bowenia are polyxylic and develop a 

 succession of imperfect cortical cambiums which form xylem zones 

 that are often more or less inverted, as in Macrozamia and Bowenia, 

 thus suggesting comparisons with such Pteridosperm genera as 

 Medullosa. 



The Cycadales and presumably the majority of the fossil cycado- 

 phytes send down a primary root, which continues as a taproot, often 

 approaching the trunk in size. All the forms have compound leaves. 

 In most genera they are simply pinnate, but in Bowenia and cer- 

 tain species of Macrozamia and Cycas the pinnules are further sub- 



