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RELATIONSHIPS. 



old stems of certain forms {Macrozamia). Also, as was first observed by Mettenius, 

 the spiral tracheitis may be torn asunder by the tension of stem increase and finally 

 become obscure ; but the scalariform tracheids continue their growth. In the 

 girdles of the young leaves the shortest measured 0.09 mm., in older stems 1.4 

 mm., and in the oldest examined 4.5 mm. There may also be an increase in many 

 of the tracheids of the secondary wood to twice the diameter seen in the cortical 

 bundles of young stems. The thin-walled parenchyma rays traversing the wood 

 and bast are successively increased as such thickening goes on. Obviously enough, 



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Fig. I 15. — Cycas neo-caledonica. X 1 1 2 . Staminate plant bearing a terminal cone. 

 (Compare with the ovulate C. media, figure 102.) 



the development of a strong anastomosing cortical bundle system must add 

 greatly to the strength of the stem and in part compensate for armor excision. 

 To say the least, it is not probable that in the absence of a secondarily strengthened 

 cortex trunks could ever reach a height of 23 meters, as does Cycas Normanbyana. 

 Conversely, the absence of cortical strengthening in the Cycadeoidese is in the 

 strictest accord with their usually shorter trunk type. 



The cortical parenchyma, like all the other ground tissues of the cycadean 

 stem, is freely traversed by gum canals. These branch freely, and may contain 

 crystalline aggregates of calcium oxalate. Stone cells often occur. 



