I 



THE FIBRO-VASCULAR BUNDLES. 



115 



the lamina, constituting its venation. Every bundle is completely surrounded by 

 fundamental tissue both in the stem and in the leaf, and is therefore isolated from 

 the rest ; the only connection between different bundles takes place at their lower 

 ends within the stem. The arrangement of the fibro-vascular system in the root is 

 strikingly different, if we compare with it in the case of stems only the original 

 structures produced by differentiation from the primary meristem, and not the 

 thickening-tissues which subsequently arise from the secondary meristem or cambium 

 (see Sect. 18). The root is usually cylindrical and very slender; and its transverse 

 section, both in Cryptogams and Phanerogams, shows, beneath the epidermis, a thick 

 layer of parenchymatous fundamental tissue, surrounding a cylindrical bundle which 

 traverses the whole length of the root. This bundle may be termed the axial cylinder 

 or Plerome; it is always sharply separated from the cortical parenchyma by an innermost 

 layer of the latter, the Bundle-sheath or Plerome-sheath. In most stems also a similar 

 sheath separates the cortex 

 from an internal cylinder 

 of tissue^ containing the 

 fibro-vascular bundles (Fig. 

 93, p. 112). It may easily 

 be recognised by the cha- 

 racter of the longitudinal 

 partitions of its cells in a 

 radial section, which, in 

 consequence of their pecu- 

 liar folding, appear, on 

 transverse section, as if 

 marked with a black dot 

 (Fig. 96, s)'. 



Within this bundle- 

 sheath is usually found in 

 thick roots a large number 

 of ribbon- shaped vascular 

 bundles arranged in a ring. 

 In each vascular bundle the 

 oldest but smallest vessels 

 lie on the outside next the 

 sheath(Fig. 96, />,/>) ; from 

 them the formation of ves- 

 sels advances centripetally, 

 so that the later-formed 

 vessels which lie nearer the centre are always larger and broader. Between any two 

 groups of vessels there always lies a bundle of phloem {ph), which not unfrequently 

 has true bast-fibres on its outer margin. The rest of the axial cylinder consists of paren- 

 chymatous tissue. 



In slenderer roots the number of xylem- and phloem-bundles in the axial cylinder 

 is commonly reduced to two or three, and in that case the former usually meet 

 in the axis, so as to form either a broad band which divides the cylinder in two, or 

 a three-rayed star of vessels. In thicker roots, on the contrary, the inner edges of 

 the vascular bundles do not usually reach the centre of the cylinder, which in these 

 cases consists of a mass of parenchymatous tissue or pith, m. Roots which, like most 



Fig. 96. — Transverse section of root of Acorns Calamus, showing the axial cylinder 

 and the surrounding cortical tissue; s bundle-sheath or plerome-sheath; pp the oldest 

 narrow peripheral vessels ; g- large broader vessels which are always younger ; ph phloem- 

 bundle ; m central parenchymatous tissue or pith ; pc pericambium. 



' See also the end of Sect. 19, and Van Tieghem, Memoire sur les canaux secreteurs; Paris, 

 1872 ; foot-note, p. 17. 



'^ [This is well shown by Van Tieghem, Recherches sur la symt^trie de structure des plantes vas- 

 culaires, in Ann. des Sci. Nat. 5th ser, 1871, vol. xiii. pi. 3-8.] 



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