252 



S1GILLARIEAE. 



middle portions are entirely destroyed ; in the inner portion there remain 

 only the transverse sections of the ascending leaf-traces, and these are 

 obtusely triangular in form and have one angle turned towards the outside. 

 We shall have to return to the character of the leaf-trace-bundles of 

 Sigillariae in concluding our account of the anatomy of the group. The 

 outer rind which bears the leaf-cushions is preserved ; it consists of 

 compact parenchyma and separates into two not very distinct layers, of 

 which the inner is composed of elongated elements disposed in rows on 

 the transverse section, the outer of ordinary isodiametric parenchyma. 

 The former is always termed in Renault's publications the ' couche 

 subereuse,' an expression which must be understood here as in Lepi- 

 dodendreae in a morphological, not in a physiological sense, being intended 

 to show the connection with a periderm. We are in 

 fact vividly reminded of the formation of periderm, 

 which we observed in Lepidodendreae. 



Of the group of Leiodermariae we are ac- 

 quainted through Renault's 1 labours with the structure 

 of Sigillaria spinulosa, of which a good many 

 specimens have been obtained at Autun. Rind and 

 wood are there usually found separate from one 

 another, and the surface of the former is in most 

 cases not preserved. The rind had been known for 

 some time, and had been named by Brongniart Dic- 

 tyoxylon before the relation of the two remains 

 to each other was cleared up by the help of some 

 fortunate discoveries, which showed them in actual 

 connection with one another and revealed at the 

 same time the characteristic features of the surface. 

 Here too there is a central cavity evidently once 

 occupied by the pith ; this cavity is surrounded 

 as in Sigillaria Menardi by a number of vascular 

 strands, which are more or less circular in form and are succeeded on the 

 outside by a much thicker mass of secondary xylem. The bundles also 

 which surround the pith are similar in structure to those of Sigillaria 

 Menardi, but they have not their regularity of form ; they vary greatly 

 in breadth and several of them are often attached laterally to one another. 

 To put the matter briefly, we cannot dismiss the impression that we have to 

 do in this case with a circular tracheal zone, which through unequal develop- 

 ment has been broken up into many separate fragments having naturally 

 therefore the form of strands. This is not so distinctly shown in Renault's 2 



FIG. 28. Sigillaria Menardi, 

 lirongn. Fragment of the trans- 

 verse section of the ring of wood. 

 The primary bundles show their 

 initial strands on the outer side ; 

 the section has encountered an 

 emerging foliar strand at a ; at 

 / is the transverse section of a 

 similar strand which has already 

 passed through the secondary 

 wood. After Renault (2), vol. i. 



1 Renault (1) and (8). 



3 Renault (8). 



