FIBBO-VASCULAB, SYSTEM. 515 



of division, and which may be grouped under the three heads of 

 dermatogen, periblem, and plerome. JFrom the dermatogen-celis the 

 root-cap, the epidermal tissues and their appendages, hairs, &c., 

 develop themselves. The cells constituting the periblem are the 

 precursors of the cortical tissues. The cells of the plerome form 

 the pericambium, procambium, cambium, and ultimately the fibro- 

 vascular bundles and pith. The pericambium is only found in 

 roots, and is merely the outer layer of the plerome (McNab). The 

 procambial cells are those which are destined to develop into the 

 libro-vascular bundles. The cambium is that portion of the original 

 plerome which is not converted into fibres or vessels, but the cells 

 of which retain their more or less spherical form and their power 

 of subdivision. The cambial cells occupy the centre of the fibro- 

 vascular bundles of Dicotyledons, between the outer phloem portions 

 and the inner xylem or vascular portions. At the extreme apex 

 these layers are not yet differentiated, but form a mass of cells of 

 equal size and degree, sometimes called the initial cells. In many 

 Cryptogams, however, the ends of the stem and of its subdivisions 

 are constituted by a single apical cell. 



The Fibro-vascular System. This system forms all the woody 

 structures of plants, which in all cases are composed of a quantity 

 of conjoined portions of cellular and vascular tissue arranged in a 

 peculiar manner, and derived originally from a definite portion of 

 the plerome called the procambium. The cells of this latter are 

 either all converted into permanent tissue (vessels, liber-cells, &c.), 

 or some of them remain in a merismatic condition, if capable of 

 division, and these form the cambium. The kind of cellular 

 tissue associated with the vessels is mostly prosenctiyma or fibrous 

 tissue ; the constituent elements wood of are called jibro-vascular 

 bundles. In Dicotyledons the fibro-vascular bundle usually con- 

 sists of wood-cells and vessels (xylem) internally, liber-cells (phloem) 

 externally, separated by cambium. The bundles are plunged in paren- 

 chyma. If the bundles are devoid of meristem or cambium they 

 are closed ; if, on the other hand, they contain cambium, the bundles 

 are called open. All woody substance appears originally in the con- 

 dition of isolated fibro-vascular bundles, which, when they remain 

 separate, form what are commonly called " fibres," and when they 

 combine together into a solid mass, form " wood." The bundles re- 

 main as " fibres" in the stems of Monocotyledons ; they are in the 

 same state in the earliest conditions of the stems of Dicotyledons ; 

 and such " fibres " form the ribs of leaves and other organs. 



That portion of the parenchyma which remains after the con- 

 version of the meristem into fibro-vascular tissue is called the 

 fundamental tissue. 



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