﻿EXISTING AND FOSSIL CYCADS COMPARED. 



20' 



peduncle in a quite horizontal position, and occupy a less and less displaced, more 

 and more vertical direction as the apex of the stem is approached. 



Structure of Corlical Bundles. — The xylem of the girdles normally faces the 

 xylem zone and the phloem the periderm. In their course through the cortex 

 from their origin in the xylem zone to their entrance into the frond bases, the 

 girdle bundles display various peculiarities of structure. In Cycas in particular 

 there is often to be noted a quite well-marked concentric and distinctly primitive 

 arrangement, a parenchymatous prism or pith being surrounded successively by 



Fig. I 14. — Cycas Rumphii. )■ 1 l - i . Plant bearing emergent young fronds with erect rachides and incurved 

 pinnules. Armor moderately persistent and scale leaves long and curling. 



the wood and cambium and a weak ring of bast. [These concentric traces may 

 possibly be mainly carpophyllar supply bundles.] 



Moreover, in addition to the cambiogenetic increase already discussed, the 

 pith and cortex undergo expansion as the growth and increase in circumference 

 of the stem proceeds ; and as the primary network of bundles remains meantime 

 permanent, the radial connections and especially the girdles of the peripheral cortex, 

 because of their unique course, must be extensible and undergo stretching and 

 increase in length continuously. Hence the presence of cells with dense spiral 

 thickenings in the tracheidal walls of the secondary wood of the girdle bundles in 



