Verticillaphyton paradoxum, a Problematical Fossil Genus of Pennsylvanian Age from Iowa Coal Balls

Abstract

The material described here consists of a single small plant axis approximately 7.0 cm in length and 3.0 mm in diameter at one end to 5.5 mm at the other. The specimen was discovered in a coal ball of Middle Pelansylvanian age from the Mich Coal mine of Oskaloosa, Iowa. While the total length of the one specimen was only around a centimeter, it seems obvious it was a fragment of a larger plant as its disapperance in the final serial peel sections at each end was due to a gradual decrease in preservation rather than alnatural termination of the structure. At its small end a transverse section of the axis shows an oval outline with dimensions of 3.0 X 2.2 mm. The stele is 1.7 X 1.2 mm with a small central pith 0.25 mm in diameter (Fig. 1, 10). The outermost tissue consists of an irregularly lobed cortex of homogen.eous collenchyma, Elled with a dark-brown opaque substance. This encloses the primary xylem of the exarch stele with several inconspicuous peripheral points of small protoxylem cells in some sections. Secondary wall thickening is annular in the protoxylem and forms scalariform to reticulate-bordered pitting patterlns in the metaxylem (Fig. 9). The pith (where present) consists of a tissue identical to the cortex. No tissues identiSable as epidermis or phloem were recognized in any of the sections observed. Serial peiel sections (in what is assumed to be an upwards direction) show that, in a space of 1.0 mm from the small end of the axis, the pith enlarges from 0.25 mm to 1.0 X 0.5 mm produci,ng a conspicuous siphonostele with the radial thickness of the xylem being reduced to 0.25 mm (Fig. 2, 11). Within the next 1.5 mm the asis increased to a maximum width of 5.5 mm while the xyIem cylinder splits into a verticil of 9 strands each of which is first protostelic and then siphonostelic through the development of a central medulla. At the same time, the axis as a whole, develops a hollow center with the disappearance of the central pith tissue (Fig. 3, 4, 12). At a slightly higher level the hollow axis becomes a verticil of free branches by a double constriction of the cortex and pith tissue surrounding each stele (Fig. 5, 13). Each branch then proceeds to become polystelic by dichotomous stelar divisions, ultimately producing