2. Vascular Cryptogamia,[37] or Acrogens. 3. Monocotyledons.[38] 4. Gymnospermous Dicotyledons.[39] 5. Angiospermous Dicotyledons.[40] [36] Plants having the fructification concealed, and of cellular structure only. [37] Plants having the fructification concealed, and with vessels, or vascular tissue. [38] Flowering plants with one cotyledon; the Endogens. [39] Plants with naked seeds; that is, destitute of a pericarp or case. [40] Plants with the seeds in a receptacle or pericarp, with a style and stigma. ON THE MODE OF INVESTIGATING FOSSIL REMAINS OF VEGETABLES. [Sidenote: INVESTIGATION OF FOSSIL PLANTS.] The distinguished authors of the British Fossil Flora justly remark, that a few isolated, and very imperfect data, exclusively afforded by the remains of the organs of vegetation, are but too often the sole guide to the class, order, or genus of the fossil plants which the geologist has to examine; hence, in most instances, a general idea only can be obtained of the nature of the original.[41] To facilitate the study of Fossil Botany they offer some practical suggestions, which have served as the basis of the following directions for the investigation of vegetable remains, and which the previous remarks will, we trust, render intelligible. [41] Foss. Flor. vol. I. p. xxvi. 1. _The Trunk, or Stem._--Examine if the wood in a transverse section be disposed in concentric circles (as _Plate V. fig 4_): if so, it belonged to an exogenous tree: if, on the contrary, the wood appears deposited irregularly in spots (_Lign. 1, fig. 4_), then the original was endogenous. If a transverse section show remains of sinuous, unconnected layers, resembling arcs with their ends directed outwards, and of a solid structure, and imbedded among looser tissue, then it belonged to an arborescent fern; see the subjoined figures (_Lign. 2_). [Illustration: Lign. 2. Sections of Fern-Stems. Transverse sections (half the diameter) of two stems of recent Arborescent Ferns, to show the zone of woody fibre disposed in arcs. This structure is seen in the silicified trunks from Chemnitz.] [Sidenote: INVESTIGATION OF FOSSIL STEMS.] If the stem be in a state of preservation that will admit of the slicing or chipping off a piece for microscopical investigation, the process described at the conclusion of this section should be employed. The following data may be thus obtained. If the structure be entirely cellular, and it can be satisfactorily ascertained that it never possessed vascular tissue, the original belonged to the _Cryptogamia_; _i.e._ to fuci, mosses, and the like.