﻿EXISTING AND FOSSIL CYCADS COMPARED. 



20I 



The Cortex. 



The cortex consists in the wide zone of parenchyma traversed by the leaf trace, 

 girdle leaf-trace and peduncle bundles, and the mucilage canals. It incloses the 

 xylein from which its bundle system arises, and is bounded exteriorly by the 

 periderm below, and above by the armor of old foliar bases in so far as not cut 

 away by the advance of the periderm. Its radial thickness is from one-third that 

 of the entire trunk (including the armorj in Cycas to as much as two-thirds in the 

 scantily armored Zamia floridana. 



Cortical Bundle Distribution. — In the ferns the leaf traces pass out from their 

 stelic origin obliquely upward into the bases of the fronds, and in the Cycadeoidete 

 there is, as has been seen, a similar more or less direct course of the leaf traces from 

 the xyleni zone out through the cortex and into the leaf bases. But in the living 

 cycads (see fig. 113) an}- such definite and primitive arrangement as might once 

 have been present is now more or less obscured. The nearest approach to the 

 simple bundle course is to be seen only at the apex of the young stem or in 



peduncle traces, and the most complex and involved 

 course in the well-grown and heavily armored trunks 

 of Dion, Cycas, etc., as first described by Mettenius (92). 

 It appears, however, that various Zamias and Stan- 

 geria are not quite so complex as these. If the bun- 

 dles of the cortex of a well-grown trunk of Zamia 

 floridana be followed by dissection out from their 

 origin in the xylem, they are found to pass almost 

 directly outward and upward at an average angle of 

 some 30 degrees for from two-thirds to rarely almost 

 the entire width of the cortex, and then curve slowly 

 upward either to the right or left through from a few 

 to most frequently as many as 50 or 60 degrees of the 

 cortical circumference, before being cut off by peri- 

 derm (see fig. 113) or entering the leaf bases, each of 

 which receives a pair from opposite directions. 



Zamia muricata has a similar cortical system; and 

 likewise in the middle portions of adult Stangeria trunks, owing in part to the 

 advance of the periderm, the leaf traces run out in the radial or nearly radial direc- 

 tion, with but little curving in the peripheral cortex. Lateral leaf-trace connections 

 seldom occur in either this or the two preceding forms. Boweuia has not been 

 carefully studied, so far as known to me. 



In describing the cortical system of the markedly complex type, the terms 

 usually used for the different courses of the leaf traces are as follows: 



(a) Radial traces (Markscheidenbiindeln of Mettenius), the bundles arising 

 singly or as a branching and more or less anastomosing system directly 

 from the xyleni zone. 

 (l>) Girdle traces, continuation of the radial trace bundles passing in circular 



course through the peripheral cortex into the leaf bases. 

 (c) Radial connections, bundle anastomoses involving the girdle traces. 



Fig. 113. — Zamia floridana DC. 



Transverse view of stem as dissected out on an in- 

 verted conical surface indined upwards about 

 35 , the angle at which the most of the leaf- 

 trace bundles rise in passing from the xylem to 

 outer cortex. Several of the bundles are plainly 

 seen to pass out all the way to the thin outer 

 pnelloderm and cork covering the stem. None 

 of the girdles deflects laterally through more 

 than an angle of 60 . as mainly due to the 

 advance of the periderm and excision of the 

 peripheral cortex, in which bundle curvature 

 is the more marked. 



