The older part of a Sigillarian stem may present an appearance 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[470], and afterwards illustrated by Zeiller[471] 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.[472] The piece of _Carica_ stem, represented in fig. 197, illustrates the danger of trusting to the disposition of leaves as 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[473]. Some authors have adopted a two-fold classification of Sigillarian stems proposed by the late Prof. Weiss[474] of Berlin, who divided the Sigillariae into (A) Sub-Sigillariae, comprising Leiodermariae and Cancellatae, and (B) Eu-Sigillariae, including _Favulariae_ and _Rhytidolepis_. Grand’Eury[475] adopts the terms _Rhytidolepis_ 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.