7° 



MORPHOLOGY OF GYMNOSPERMS 



some trunks bearing but little. Among the cycads, certain genera, 

 as Dioon, Encephalartos, etc., produce long one-celled or two-celled 



hairs which may be regarded as re- 

 placing in a general way the ramen- 

 tum of ferns and of Bennettitales, 

 but they are so different from the 

 scales that make up the ramentum 

 that the latter must be regarded 

 as a very distinct feature of the 

 Bennettitales. 



VASCULAR ANATOMY 



The anatomy of the stem is 

 very much like that of the stem of 

 Cycadales. A transverse section 

 shows a very thick cortex, usually 

 a comparatively thin vascular cylin- 

 der, and a large pith with mucilage 

 or gum canals or reservoirs. The 

 vascular cylinder is made up of 

 collateral endarch bundles, so that 

 the most advanced type of gymnosperm cylinder had been attained; 

 in fact, it had been attained by some of the Cycadofilicales. In 

 Cycadeoidea Jenneyana and C. ingens the vascular cylinder is so 

 massive (22) as to suggest the cylinder of Cordaitales, being made 

 up of successive woody zones. Among the cycads there is sometimes 

 a succession of vascular zones which arise from a succession of 

 cortical cambiums; but in these species of Cycadeoidea the cylinder 

 is so compact that it suggests a persistent primary cambium. 



The vascular cylinder of Bennettitales and of Cycadales is regarded 

 by WoRSDELL (23) as giving clear evidence of its derivation from the 

 MeduUosa type of cylinder among the Cycadofilicales. His proofs 

 are largely obtained from investigations of the cotyledonary node 

 and strobilar axis of certain cycads, polystely having been found in 

 the cotyledonary node of a species of Encephalartos, and a very irreg- 

 ular orientation of bundles in the peduncle of Stangeria. According 

 to this view, the cycadean cylinder has been built up by the regular 



Fig. 58. — Cycadella ramentosa: 

 transverse section of the ramentum of 

 the leaves; X66. — After Wieland (22). 



