THE FIBRO-VASCULAR BUNDLES. 
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. i8). The root is usually cylindrical and very slender; and its transvei-se 
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 1 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, sY\ 
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, p^p) ; 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 
^ 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 symetrie de structure des plantes vas- 
culaires, in Ann. des Sei. Nat. 5th ser., 1871, vol. xiii. pi. 3-8.] 
I 2 
Fig. 96.— Transverse section of root of Acorns Calmnus, showing the axial cylinder 
and the surrounding cortical tissue ; -r bundle-sheath or plerome-sheath ; the oldest 
narrow peripheral vessels ; ^ large broader vessels which are alwaj's younger ; J>h phloem- 
bundle ; 7n central parenchymatous tissue or pith ; /c- pericambium. 
