76 INDEFINITE FIBRO-VASCULAR BUNDLES. 
The characteristic peculiarities thus found to exist in the in- 
ternal appearance and growth of these three kinds of stem are 
due to corresponding differences in their component parts, or, as 
they are commonly called, their fibro-vascular or vascular bundles. 
Thus, the fibro-vascular bundle of an Exogenous stem (fig. 179) 
consists in the first year of growth of a layer of spiral vessels 
(s, s’, and fig. 180, sv), surrounding the pith (p’, and fig. 180, p) ; 
on the outside of this layer there are subsequently developed, in 
perennial plants, pitted vessels (p, p, and fig. 180, d) and wood- 
cells (w’, w, and jig. 180, w), which together form the wood. But 
in herbaceous plants annular and reticulated vessels are also found 
Fie. 179; 
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Fig.179. Radial vertical section though an indefinite fibro-vascular bundle 
from the stem of the Sunflower. p’. Pith. s,s’. Spiral vessels. w’, w. Wood- 
cells. p, p. Pitted vessels. c. Cambium. st, st. Sieve tubes. ph. Liber- 
cells, m. Bundle-sheath, CO’. Cellular layers of the bark. After Prantl. 
intermixed with the wocd-cells. The wood is covered externally 
by a layer of vitally active or generating cells (figs. 179, c, and 
180, c), called the cambiwm (see page 88), on the outside of which 
are the liber (figs. 179, st, ph, and 180, 1), the cellular parts of 
the bark (figs. 179, C’, and 180, ¢ e), and the epidermis (jig. 
180, ¢). The different kinds of tissue which are placed within 
the cambium, or cambium layer as it is frequently termed, form 
what has been called the «xylem or woody portion of the bundle, 
and those outside the cambium forming the liber, that portion 
which has been termed the phloém; so that the fibro-vascular 
bundle has the pith (jig. 180, p), on its inner surface, and is 
covered externally by the cellular layers, ce, of the bark. In the 
stem of some plants, as in the above, a single special layer of cells, 
