﻿TRUNK STRUCTURE. 



65 



explained as a much-thickened outer segment of a discontinuous bundle sheath, 

 which has encroached upon and taken the place of the outer thin- walled phloem. 



Two counterparts of the bundles just described present parallels of fundamental 

 importance. Firstly, the transverse section of part of the stem of Lyginodetidron 

 Oldhamium, figured by Scott (Fossil Botany, p. 315), is a prototype which pre- 

 sents identically the arrangement and development of mesarch xylem seen in C. 

 dacotensis. Secondly, the most complete replica of the form of cycadeoidean bundle 

 here desciibed, so far as known to the writer, is to be observed in the petioles 



-Cycadeoidea dacotensis. 

 Transverse sections of vascular bundles of leaf bases. 



X60. 



(A, B). Typical bundles as cut several centimeters be- 

 yond the cortex in leaf bases surrounding a laterally- 

 borne bisporangiate stTobilus. 



'C). From S. 506. A bundle from a much younger 

 leaf base near a young fruit close to the summit of the 

 trunk, about a centimeter distal to the cortex. The 

 extensively developed centrifugal xylem and the proto- 

 xylem is the only portion silicihed. 



cf. Centrifugal xylem ; cp, centripetal xylem : px, pro- 

 toxylem ; m. medullary ray ; p, phloem ; s, sclerenchyma. 



S.S0b. 



of the carpellary leaves of Cycas celebica. It is believed that when an adequate 

 series of sections, so cut as to permit exact comparison in each case, has once been 

 prepared from both the fossil and existing cycads, a complete parallelism will be 

 evident. Nor is it uncalled for to remark that the more the writer has examined 

 these two groups of gymnosperms the more fixed has become his belief that the 

 pronounced structural agreements exhibited by them can never have been the result 

 of homoplastic development. They are undoubtedly sprung from the same original 

 fern stock, and their relationship is ancient and profound. The variation they show 

 is due to the fact that, in the evolution of both the existing and the extinct groups 



