686 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
The smaller specimens, such as those which were first described by Hick and CasH 
under the name Myriophylloides,* but have since been united with Astromyelon,t 
were no doubt branches of the larger organs, as will be shown below. 
Structure of the Roots. 
We have now to consider in detail the structure of these appendages, with special 
reference to those points by which their morphological nature has been established. 
That they were quite different from the ordinary branches of a Calamitean stem 
became obvious as soon as their organization was understood.{ Their characteristic 
lacunar cortex, the entire absence of nodes, the usually solid pith, and the absence of 
canals accompanying the vascular bundles, all serve to distinguish them. That these 
organs were of the nature of roots could, however, only be established by a minute 
investigation of their structure and mode of development, so far as the latter could be 
ascertained. We hope that, by our observations, we have been able to add something 
to the evidence already brought forward by Renauxr, who recognizes most of 
these organs as true roots, though in certain cases he prefers to regard them as 
* stolons.’’§ 
The characters (likely to be traceable in petrified specimens) on which we have to 
depend in distinguishing a root from a stem, are the following :— 
1. Centripetal development of the primary xylem. 
2. Alternate arrangement of the primary groups of xylem and phloém. 
3. Endogenous mode of origin of the organ itself, and of its branches. 
4. Absence of nodes. 
Though no one of these characters is necessarily conclusive by itself, the sum of 
them is sufficient to determine with certainty the root-nature of the organ. 
In describing the structure it will be best to begin with the typical specimens, 
possessing a well-marked medulla, such as those which have been found in direct con- 
nection with Calamitean stems. We will afterwards pass on to the more minute 
specimens, many of them destitute of any evident pith, which we regard as the 
rootlets, or finer branches, of the same organs. 
The largest specimen in the collection is one figured in a previous memoir.|| This is 
somewhat compressed, and measures, in its decorticated condition, about 25 millims. 
in greatest and 11 millims. in least diameter. The pith is hollow, and this is the only 
specimen in which, from the definite inner limit of the persistent peripheral portion of 
the medulla, it appears probable that the latter was really fistular during life. In 
* Loc. cit., ‘ Proc. Yorkshire Geol. and Polytechnic Soc.,’ vol. 7, 1881. 
+ By Wituiamson, * Organization,” Part XII., 1883. 
t Wicuramson, loc. cit., Parts IX. and XII. 
§ Loe. cit., “ Flore fossile d’Autun,” &c., Part II., Plates 55, 56, 57, 59, 60. 
| Witutamson, * Organization,” Part IX., Plate 19, fig. 5; O.N. 1334. 
