[101] The reader intending to make fossil botany his particular study, should refer to the original memoir, and become familiar with the facts and inferences so admirably enunciated by the author; not only for the illustration of the structure of the tribe of plants under consideration, but as a valuable exemplification of the manner in which all such inquiries should be conducted. See _Archives du Muséum d'Histoire Naturelle_, tom. i. Paris, 1839. [Illustration: Lign. 35. Silicified Stem of Sigillaria elegans. _Autun. France._ (_M. Adolphe Brongniart._) Fig. 1.--A transverse section of the silicified stem, from Autun; _nat._ _a._ Situation of the medullary tissue, occupied by coloured silex. _b._ Zone composed of bundles of vessels, forming the woody tissue. _c._ Band of cellular tissue _d._ Space between the ligneous cylinder and the bark, with no evident structure, but originally occupied by the external cellular tissue. _e._ Zone of indistinct cellular substance. _f._ External cortical envelopment, or bark. 2.--Portion of one of the vessels of the medullary tissue, as seen in a longitudinal section, (× ×.) 3.--Portion of a spiral vessel of the same tissue. (× ×.) ] The annexed figure (_Lign. 35_) is an outline of the specimen, of the natural size; but this sketch is a mere plan or diagram, for it is impossible without the aid of colour to convey a faithful idea of the original. The student should observe, that when mineral matter has permeated the stems of plants, the vascular tissue is often so well preserved, that one such specimen affords more important information, than hundreds of examples in which the form alone remains. [Sidenote: SIGILLARLA ELEGANS.] The external surface of this specimen possesses the characteristic markings of the insertion of the leaf-stalks of _Sigillaria elegans_. The internal organization, as seen in the transverse section, is as follows:-- _a._ The centre, filled with silex; it exhibits no traces of structure. _b._ The zone which surrounds the interspace on which this letter is placed, is composed of bundles of vascular tissue. A portion of one of these bundles, highly magnified, is represented in _Plate V. fig. 7_. The _inner_ circle of this zone, indicated by the _convex undulating line_, is made up of medullary vascular tissue; the external circle is divided by rays, and is composed of woody fibre, constituting a ligneous cylinder. One of the spiral vessels (_fig. 3_), and another showing a remarkable difference of structure in a short space (_fig._ 2), as seen in a longitudinal section of the medullary tissue, are figured in _Lign. 35_.