FIBRES. 



^33 



oblique slit-like pits, and these alternate with thick bands of wall of equal height. 

 The epidermal elements overlying this fibrous covering show, as was intimated on 

 p. 71, the same structure. 



For the more minute structure of the thickening masses, those general rules hold 

 which apply to the structure of the cell-walls ^ In the fibres united into bundles, 

 and those in the bast, there may often, but not always, be distinguished three dif- 

 ferent concentric systems of layers, or sheaths, the outermost limiting layer, an inner 

 layer, and a middle layer, which is usually much broader and softer. The fibres oT 

 the Apocyneae and Asclepiade^e are excellent examples of the slriation and areola- 

 tion of the wall. — The branched fibres in the leaf of Sciadopitys, Dammara, Araucaria 

 imbricata, Nymphseaceae, and especially the colossal spindle-fibres made known by 

 Hooker ^, which lie scattered in all parts of Welwitschia mirabilis, are characterised 

 by numerous crystals of calcium oxalate which are imbedded in the outer layers of 

 their walls, and which, especially in Welwitschia, attain a considerable size (Fig. 55). 



The wall of the sclerenchymatous fibres is lignified, to a very variable extent 

 according to the special case: of the bast-fibres used in manufacture, e.g. according 

 to Wiesner, those of Flax, Hemp (light yellow with aniline and sulphuric acid), and of 

 Hibiscus cannabinus turn blue (of difi"erent shades) with iodine and sulphuric acid, 

 and with aniline and sulphuric acid not at all or hardly yellow ; with the preparation of 

 iodine the fibres of species of Corchorus, Sida retusa, Urena sinuata, &c. turn yellow 

 or brown, with aniline and sulphuric acid yellow. In the Ferns and Rhizocarpeae the 

 fibres of this category have also the above-mentioned (p. 121) characteristic dark- 

 brown colour. In fibres in the bast Sanio ' often found the especially thick inner 

 layer of the wall cartilaginous and gelatinous, and that it swelled in water, and 

 turned violet with Schultze's solution or solution of iodine in potassium iodide 

 (e.g. in Cytisus Laburnum, Morus alba, Ulmus suberosa, Celtis australis, Ficus Syco- 

 morus, Robinia pseudacacia, Gleditschia triacanthos, Quercus pedunculata, Passiflora 

 suberosa) ; this phenomenon also occurs in various modifications in the fibrous 

 elements of the secondary wood of Dicotyledons, and will be described with the other 

 properties of these elements in Chap. XIV. Conversely it sometimes happens that 

 sclerenchymatous fibres develope from originally collenchymatous cells, in which 

 case the inner layers of the walls become hard and lignified, while the outer retain the 

 original collenchymatous character: e.g. in the bands accompanying the vascular 

 bundles of Eryngium planum, and Astragalus falcatus *. 



As regards the contents of the fibres we must refer to what was above stated 

 (p. 126) for the sclerenchyma generally. The granular constituents or remnants of 

 the contents, enclosed by many fibres with a larger cavity, e.g. the enlarged parts of 

 those of the Asclepiadeae and Apocyneae, have repeatedly led to the view that the 

 bast-fibres contain the characteristic /a/ex, which exudes on cut surfaces in the 

 Asclepiadeae, Euphorbiaceae, &c., a false idea, which will be di cussed in Chap. VI. 



In the majority of cases, and in all those which have been hitherto noticed, the 



* See Hofmeister, PflanzenzpUe, § 27, 28. 



^ Trans. Linnsean Society, vol. XXIV ('Spicular cells'). 



^ Bolan. Zeitg. 1863, p. 105. — Ibid, i860, Taf. VI. 15 and 16. 



* Schwendener, /. c. p. 5. 



