Manchester Memoirs, Vol. lix. (191 5), No. 3. 27 



longitudinal growth to allow of surface for the insertion 

 of the leaves and roots, there would be no progressive 

 increase of the cortical lobes. Both the shoot stele and 

 the rhizophoric stele would be clothed with their respec- 

 tive cortex, the form of the plant being given by that of 

 the stele, and the lobes therefore corresponding in posi- 

 tion with the grooves of the Isoetes plant as we know it. 12 

 Such a plant would have a remarkable resemblance to 

 Pleuromeia, in that the cylindrical stem would end below 

 in a lobed region, without evident localised growing 

 points, bearing all the roots of the plant exogenously. If, 

 further, we suppose the growth of the rhizophoric region 

 of this P leurovteia-\'\ke plant localised near the tips of 

 the lobes and ceasing along the lower portion of the 

 convexities, long cylindrical rhizophoric axes might be 

 developed. The resulting plant would agree with Lepido- 

 dendron or Sigillaria in the possession of a cylindrical 

 main stem with a system of diverging root-like organs 

 forming a stigmarian base. 



Such comparisons must of course be speculative but 

 they are justified by the close and detailed resemblances 

 in the root-scars and root-insertions, in the structure of 

 the roots, and the orientation of their structure towards 

 the growing region in Isoetes to the corresponding feature 

 in the Lepidodendrese. The root bearing basal region of 

 the latter requires morphological explanation Unfortu- 

 nately we have no knowledge of its development nor 

 indeed of the structure of the basal portion of the plant 

 at the junction between stem and rhizophoric region. 



1 2 Superficial comparisons are so often made between the lobing of the 

 stock of Isoetes and the lobes of the root-bearing base of Pleuromeia or the 

 Lepidodendreoe, that it may be vi ell to emphasise the fact that such com- 

 parisons are erroneous. It is the grooves of the Isoetes stock, beneath which 

 the lobes of the rhizophoric stele lie, that correspond morphologically to the 

 lobes of Pleuromeia. 



