330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



fossils, particularly in the underclays beneath the co:il seams. In 

 complete specimens the main axis is seen to divide into four main 

 forks which run horizontally and branch dichotomously. The sur- 

 face is smooth or irregular, wrinkled, and covered with well-spaced 

 umbilicate scars from which, in well-preserved material, cylindrical 

 radiating rootlets are seen to diverge. These are slightly constricted 

 at the base and lack root hairs. These are so common that the under- 

 clays are often called Stigmaria clays, and they are the pest of stu- 

 dents of petrified material, since they are found penetrating the 

 tissue of other plants that formed the substratum in the Carbonifer- 

 ous swamps. The main axis consists of a central pith, a ring of 

 centrifugal wood accompanied by phloem and surrounded by a cortex 

 with abundant periderm. Sometimes no distinction between pri- 

 mary and secondary wood is observable, while in other specimens 

 centripetal primary wood is present. The rootlets show a different 

 anatomical arrangement, each containing a monarch stele, with radial 

 vascular strands connecting the protoxjdem with groups of cortical 

 tracheids. They arise from the inner margin of the primary wood, 

 although the outer cortex is continuous with that of the main axis. 



Stigmaria have frequently been found attached to both Lepidoden- 

 dron and Sigillaria trunks. There has been much discussion regard- 

 ing the morphological nature of Stigmaria, since they do not conform 

 to the usual morphology of true roots, but whatever their morphologi- 

 cal homologies, physiologically they are roots. 



The fructifications of the Lepidodendraceae have been described 

 under a variety of generic names, the most common being Lepido- 

 strobus. Lepidostrobus had an axis similar anatomically to a vege- 

 tative twig and bore numerous spirally arranged sporophylls, each 

 bearing a single very large radially elongated sporangium on its 

 adaxial surface. The sporophyll had an upturned blade and these 

 formed an imbricated protective surface for the cone. Between the 

 sporangium and the blade a ligule was present. It seems probable 

 that the Lepidodendrons were always heterosporous, the two kinds 

 of spores being produced on different parts of the same cone or upon 

 different cones. The microspores were small, tetrahedral in form, and 

 very plentiful. The megaspores were relatively very large, few in 

 number, tetrahedral in form, with a hairy, surface and commonly 

 opening by apical flaps. The prothallus within the megaspore is oc- 

 casionally preserved and even archegonia have been recognized. In 

 another cone genus, Spencerites, the sporangia were united to the 

 sporophyll by a distal neck and the spores were winged. In some 

 Lepidodendron cones large masses of sterile tissue are developed 

 within the sporangia suggesting a vestigial sporangiophore, and in 

 Mazocarpon the large sausage-shaped megaspores are imbedded in a 

 solid parenchymatous tissue. 



