Vegetable Organization.--As fragments of the stems, trunks, and branches, are very often the only vestiges of fossil plants, a knowledge of the characters by which the principal divisions of the vegetable kingdom may be distinguished by their internal structure, is indispensable to the successful investigation of the Flora of the ancient world. Although I have treated of this subject in the Wonders of Geology, (_Wond._ p. 694,) it will here be necessary to present the student with more ample details. The excellent introductory botanical works of Dr. Lindley, and Professor Henslow, convey full information on this, and every other department of the science, and should be consulted by those who intend to make this branch of Geology their particular study. For the general reader, and amateur collector, the following brief notice of a few obvious essential characters of vegetable organization, may perhaps afford sufficient information, to enable them to understand the principles on which the successful investigation of the nature and affinities of fossil plants must be conducted. [Illustration: Lign. 1. Sections of Recent Vegetables; illustrative of their internal organization. (From _Dr. Lindley_.) Fig. 1.--Longitudinal Section of Coniferous Wood. _a._ The Ducts, _b._ Spiral Vessels, _c._ Glandular vessels. 2.--Transverse section of a dicotyledonous stem. _a._ Pith, or central column. _b._ The bark. _c._ Medullary rays. _d._ Vascular tissue between the medullary rays. 3.--Elongated cellular tissue, forming the medullary rays. 4.--Transverse section of a monocotyledonous stem. ] [Sidenote: STRUCTURE OF VEGETABLES.] Every plant is essentially an aggregation of cells;[35] and the most simple forms of vegetation consist of a congeries of cells (_cellular tissue_) of the same kind, and have no visible fructification; such are the sea-weeds (_algæ_, _conferæ_, &c.), mosses, and lichens. In the more complex tribes the cells become variously modified, are elongated into tubes or vessels (_vascular tissue_), some of which possess a spiral structure, and others have their sides studded with little glands. The vascular tissue consists of two kinds of vessels. 1. The _spiral vessels_ or _tracheæ_: these are membraneous tubes, with conical extremities, having within a coil of elastic fibre spirally twisted, and capable of being unrolled (_Lign. 1, b._). 2. The _ducts_; which are a modification of the structure of the spiral vessel; their extremities are rounded or conical, and their sides marked with transverse lines, rings, or bars. Their functions appear to be different from those of the spiral vessels, and they are found in situations where the latter never occur.