240 



PLANT MORPHOLOGY 



Sigillaria^ (Fig. 19G). Both bore numerous narrow simple leaves that, 

 upon falling, left characteristic scars on the stem. The stems of Lepido- 

 dendron were dichotomously and freely branched and the leaves were 

 frequently up to 15 or 20 cm. long. The stems of Sigillaria were slightly 

 or not at all branched and the leaves, in some species, reached a length of 

 1 m. The leaves of all the Lepidodendrales had a single vascular bundle 

 and in all of them a ligule, deeply sunken in a pit, was present. In both 



Fig. 197. Transverse section of stem of Lepidodendron wunschianum, an ectophloic 

 siphonostele. The central pith is surrounded, in turn, by a narrow cylinder of primary 

 wood, an extensive zone of secondary wood, and an outer layer of bark. 



Lepidodendron and Sigillaria the base of the main stem was attached to 

 four descending branches (rhizophores) that spread out horizontally and 

 underwent repeated forking. They were covered with roots. 



The stem was either a protostele or, more commonly, an ectophloic 

 siphonostele (with the phloem outside the xylem). A primitive feature 

 was the presence of exarch primary wood. The stem was characterized 

 by marked secondary thickening (Fig. 197). The stem of Sigillaria 

 sometimes reached a diameter of 2 m. 



^ Plant fossils usually occur as detached organs or fragments. Only rarely is one 

 part of the plant found attached to another part. Until such connections are found, 

 detached organs of the same kind are placed in a "form genus." For example, 

 Lepidodendron was originally a stem genus. Its leaves were placed in the form genus 

 Lepidophylluyn and its cones in Lepidosirobus. Its root-bearing parts, indistinguish- 

 able from those of Sigillaria, are included in the form genus Stigtnaria. 



