372 STKUCTURB OF PHANEROGAMIC PLANTS. 



It is not yet known what office the raphides fulfil in the economy 

 of the Plant ; or whether they are to be considered in any other 

 light, than as non-essential results of the vegetative processes. 

 For as all these processes require the introduction of mineral 

 bases from the soil, and themselves produce organic acids in the 

 substance of the plant, it may be surmised that the accidental 

 union of the components will occasion the formation of raphides 

 wherever such union may occur ; and this view is supported by 

 the fact, that the late Mr. E. Quekett succeeded in artificially 

 producing raphides within the cells of "rice paper" (§223), by 

 first filling these with lime water by means of the air-pump, and 

 then placing the paper in weak solutions of phosphoric and oxalic 

 acids. The artificial raphides of phosphate of lime were rhombo- 

 hedral ; while those of oxalate of lime were stellate, exactly re- 

 sembling the natural raphides of the Rhubarb.' 



231. A large proportion of the denser parts of the fabric of the 

 higher Plants, is made up of the substance which is known aa 

 Ligneous Tissue, or Woody Fibre. This, however, can only be 

 regarded as a very simple variety of the " cellular ;" for it is com- 

 posed of peculiarly elongated cells (Fig. 171, a a), usually pointed 

 at their two extremities so as to become spindle-shaped, whose 

 walls have a special tendency to undergo consolidation by the 

 internal deposit of sclerogen. It is obvious that a tissue consist- 

 ing of elongated cells, adherent together by their entire length, 

 and strengthened by internal deposit, must possess much greater 

 tenacity than any tissue in which the cells depart but little from 

 the primitive spherical form ; and we accordingly find "Woody 

 Fibre introduced, wherever it is requisite that the fabric should 

 possess not merely density, but the power of resistance to tension. 

 In the higher classes of the Vegetable kingdom, it constitutes 

 the chief part of the stem and branches, where these have a firm 

 and durable character; and even in more temporary structures, 

 such as the herbaceous stems of annual plants, and the leaves 

 and flowers of almost every tribe, this tissue forms a more or less 

 important constituent, being especially found in the neighbor- 

 hood of the spiral vessels and ducts, to which it afibrds protection 

 and support. Hence the bundles or fasciculi composed of these 

 elements, which form the skeletons of leaves, and which give 

 " stringiness" to various esculent vegetable substances, are com- 

 monly known under the name of fibro-vascular tissue. In their 

 young and unconsolidated state, the ligneous cells seem to con- 

 duct fluids with great facility in the direction of their length ; 

 and in the Coniferous tribe whose stems and branches are desti- 



' The materials of the above paragraph are derived from the excellent section on 

 this subject in Prof. Quekett's "Lectures on Histology." Besides the vegetables therein 

 named as affording good illustrations of different kinds of Raphides, may be ttientioneil, 

 the parenchyma of the leaf of Jgrivc, Moe, Cycas, Encephalartos, &c., tlie cuticle of the 

 bull) of the Hyacinth, Tulip, and Garlic (and probably of other bidbs), the bark of tlie 

 .Spple, Cascarilla, Cinchona, Lime, X/)nisl, and many other trees, the pith of Eleagnvs, 

 and the testa of the seeds of Anagallis and the Elm. 



