﻿80 VEGETATIVE FEATURES. 



which the wood zone, though not nearly so broad as that above described, is very 

 compact, with medullary rays as narrow as in conifers. In this instance, too, it is 

 not finally decided what the true structure is — whether there has been accretion of 

 secondary wood from a persistent cambium, or formation of one or more anomalous 

 zones in compact order. 



If, however, as we suppose, a heavy wood zone was really developed in some of 

 the Cycadeoidese from successively formed cambiums, there is afforded yet another 

 fundamentally significant vegetative similarity between the two great cycadean 

 groups. Such a fact would go far toward removing the last barrier in the way of 

 regarding these groups as homogeneous, and indicating that their separation as two 

 great groups would be subversive of the most approved methods of classification. 

 For, indeed, if the evidence were as fragmentary as it was a half dozen years ago, and 

 such structural similarities were ascribed to homoplastic and independent origin, 

 would it not be at the expense of that other claim of paleobotanists that trunk 

 structures are conservative and persistent? It is on this very idea that much depend- 

 ence has, as we believe, been justly placed on the investigation of fossil plant stems. 



So far as the trunks of C. ingens, which occur so closely associated with C. 

 Jenneyana at Black Hawk, are concerned, no additional facts bearing on the present 

 question have yet been observed. These trunks of immense size and symmetrical 

 elliptical form, so much recalling that of Macrozamia cylindracea of tropical Aus- 

 tralia, have a large medulla with a single inclosing cylinder of collateral bundles, 

 usually of great size, just as the trunks are very large, as well seen in trunk 614. 

 In this fragment of an enormous specimen nearly as large as the type (plate 1) the 

 single woody cylinder, as seen at a distance of 37 cm. above the base, is composed 

 of very heavy collateral bundles 1 cm. in lateral or tangential breadth, with the xylem 

 2.5 cm. and the phloem 1.5 cm. in radial thickness. Toward the base of the trunk 

 the woody cylinder appears to increase to 8 cm. in thickness, but the tissues are too 

 much chalcedonized to show structure clearly or indicate that sectioning would be 

 profitable; although it is probable that the entire thickness belongs to the original 

 single bundle ring. Similar features and development of an immense initial bundle 

 ring are also to be observed in the fine trunk base (trunk 117). 



