FIBRES. 79 



CHAPTER XIX. 



WOOD (LIBRIFORM) FIBRES AND BAST (LIBER) FIBRES. 



These fibres belong to the prosenchyraatous series of tissues. 

 This series embraces those cells which at maturity lose their 

 nuclei and protoplasm, and, therefore, their distinctively cellu- 

 lar character, and have their walls thickened by secondary 

 deposits. They sometimes contain starch and traces of pro- 

 teid matter, but take no active part in the nutritive processes 

 of the plant. They serve it mainly for strengthening or sup- 

 porting, and hence have been called mechanical tissues. They 

 are serviceable also in conducting the sap. The elements of 

 these tissues are, for the most part, elongated and oblique-ended 

 or taper-pointed. Among the shorter cells transitions occur. 

 Between them and sclerotic parenchyma, and between the 

 fibrous forms and collenchyma, every gradation may be found. 



Bast and wood fibres are found in the so-called fibro-vascular 

 bundles of the plant. These bundles constitute the fibrous 

 framework of the plant, corresponding somewhat to the bony 

 skeleton of the human body. In the leaf they are the system of 

 veins, and in the stem and root the tough resistant portions. 

 Their function is partly to give strength and partly to conduct 

 the fluids of the plant. The cells composing the bundles, there- 

 fore, for the most part, have their walls thickened and are 

 elongated in the direction of the length of the organ bearing 

 them. They belong chiefly to the prosenchymatous series, 

 although other tissues are commonly included in the bundles. 

 In some plants, as the stem of the Indian Corn and the petiole 

 of the Plantain, the bundles may be readily separated in the 

 form of tough, stringy masses from the softer surrounding 

 tissue. (Break the petiole of a Plantain leaf and note the 

 tough threads protruding from the broken end. These are the 

 bundles). 



Although a number of kinds of tissues are usually found in 

 the fibro-vascular bundles, only two kinds are really essential, 

 namely, ducts (and tracheids, which may be regarded as im- 

 perfectly-formed ducts) and sieve cells. These and their asso- 

 ciated tissues always constitute separate longitudinal portions 

 of the bundle. The nature of ducts and sieve cells will be ex- 

 plained later. The portion of the bundle to which the ducts 

 belong is called the xylem, which mea.ns the wood, and that to 

 which the sieve cells belong is called the phloem or bast (phloem 

 means bark). The reason for calling this the bast or bark is 

 the fact that the inner bark of gymnosperms and dicotyledons, 

 also called the bast layer, is composed of the phloem portions 

 of the bundles, which are arranged in a circle. Within these 



