XVl] SIGILLARIA 203 



The older part of a Sigillarian stem may present an appear- 

 ance very different from that of the younger shoots. The 

 leaf-cushions may be stretched apart as the result of elongation 

 and increase in girth, while in some cases the arrangement of the 

 leaf-scars may vary on the same axis as the result of inequalities 

 in growth or changing climatic conditions. The contiguous 

 arrangement of the leaf-scars and narrow cushions characteristic 

 of the Clathrarian form of stem, as was first demonstrated by 

 Weiss\ and afterwards illustrated by Zeiller^ and Kidston, may 

 be gradually replaced (on the same specimen) by a more distant 

 disposition of the leaf-scars separated by a smooth intervening 

 surface of bark. The specimen of S. Brardi reproduced in part 

 in fig. 203, and first figured by Kidston, affords an example 

 of three "species" on one piece of stem, S. Brardi Brongn. 

 S. denudata Goepp. and S. rhomboidea Brongn.' 



The piece of Carica stem, represented in fig. 197, illustrates 

 the danger of trusting to the disposition of leaves aa a specific 

 criterion. 



Similarly, in the ribbed forms the degree of separation of 

 the leaf-scars is by no means uniform in a single species ^ 

 Some authors have adopted a two-fold classification of Sigil- 

 larian stems proposed by the late Prof. Weiss' of Berlin, who 

 divided the Sigillariae into (A) Sub-Sigillariae, comprising 

 Leiodermariae and Cancellatae, and (B) Eu-Sigillariae, in- 

 cluding Favulariae and Rkytidolepis. Grand'Eury' adopts the 

 terms Rkytidolepis and Leiodermaria for ribbed and smooth 

 stems respectively, the type to which the name Clathraria was 

 applied by Brongniart being in some cases at least the young 

 form of Leiodermarian stems. While recognising the artificial 

 distinction implied by such terms as Rhytidolepis, Leiodermaria, 

 and other sub-generic titles, we may conveniently speak of the 

 two main types of Sigillaria stems as ribbed and smooth. 



Still older stems of Sigillaria are not uncommon from which 

 the leaf-scars and other superficial tissues have been exfoliated, 

 leaving exposed a longitudinally fissured surface of secondary 

 cortex characterised by pairs of considerably enlarged parichnos 



1 Weiss, C. E. (88). ^ Zeiller (89). » Kidston (01) p. 94. 



* Seward (90=!). ' Weiss, C. E. (89). « Grand'Eury (90) A. 



