﻿EXISTING AND FOSSIL CYCADS COMPARED. 



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(a) In Zamza, Dion, Stangeria, Ceratozamia, and Microcycas (?) the large pith 

 is inclosed by a thin cylindrical and more or less open network or trellis of anas- 

 tomosing collateral bundles, followed by a conspicuous cortical region, as shown in 

 figure 113. There is in this form of stem no further formation of cambium and 

 no strongly marked secondary thickening. The phloem is usually well developed, 

 and often approaches or even ecpials the xylem in amount. In this monoxylic 

 stem, or in single woody cylinders of the polyxylic stem, the main structural features 

 are as follows : 



In transverse sections the xylem incloses the pith as a ring more or less evenly 

 divided by broad mesh rays into bundles or segments made up of narrow wedge- 

 shaped groups of cells, which are in turn separated from each other by few-seriate 

 medullary- rays. The subdivision of the xylem segments into these groups, which 



Fig. I 10.— Encephalarlos Altensteinii. X 



Low-growing columnar and heavily armored 



trunk with few scale leaves. 



begin next the pith with a breadth of one or two cells and widen sometimes to four 

 or more cells broad, is only a fairly regular one. External to the xylem is the well- 

 marked cambium, followed by the secondary phloem, which consists in a series of 

 bundle segments precisely corresponding to those of the xylem and completing 

 the collateral bundles of which the vascular zone is composed. The phloem seg- 

 ments, like those of the xylem, are divided by few-seriate pith rays into rather 

 regular 2 to 4 seriate rows of cells made up of thin-walled, small-celled elements 

 in the main trunk of Zamia floridana, etc., and Stangeria. But in the tap-root of 

 Z. floridana and in the trunks of Cycas, Dion, Encephalarlos, Macrozamia, and 

 doubtless most cycads, as likewise in the Cycadeoideas, sclerenchymatous elements 

 are more or less numerously and regularly interspersed among the row cells, thus 

 adding much strength to the stem. In fact, it is not always easy, under low powers, 

 to distinguish in the older stems of these forms the lignified fibrous elements of 

 the phloem from the tracheids of the xylem. On the outer side may lie the crushed 



