Si ee 
DEFINITE FIBRO-VASCULAR BUNDLES. ri 
termed the bundle-sheath (fig. 179, 1), forms the innermost layer 
of the cellular portions of the bark, and thus investing the fibro- 
vascular bundle. In these bundles the growth of the different 
parts is progressive, the inner part of each being first formed, 
and growth gradually proceeding to the outside, and as they 
always contain a cambium layer they are capable of further 
growth, and thus form periodically new layers of xylem and 
phloém, and are therefore called indefinite or open fibro-vascular 
bundles. It also necessarily follows from the cambium layer 
Fie. 180. Fie. 181. 
/ 
Fig. 180. Transverse section of an indefinite fibro-vascular bundle of an Exo- 
genous stem (Melon), p. Pith. sv. Spiral vessels. m7. Medullary ray. 
w. Wood-cells. d. Pitted vessels. c. Cambium layer. J. Liber or phloém. 
ce. Cellular portions of the bark. e. Epidermal tissue. Fig. 181. Trans- 
verse section of a definite fibro-vascular bundle of an Endogenous stem 
(Palm), the upper portion being directed to the centre. w. Wood-cells. 
sv. Spiral vessels. c. Cambium-like cells. d. Pitted vessels. p. Paren- 
chyma (ground tissue), surrounding the bundle. 7, Liber-cells, 
being placed between the xylem and the phloém, that the layers 
of increase to these parts of the bundle are in continuity with 
the previous ones. 
In Endogenous stems the fibro-vascular bundles (fig. 181) 
consist internally of wood-cells, w, and spiral vessels, sv; on the 
outside of which other spiral vessels are formed, as well as 
pitted, d, and other vessels; these are succeeded by a number 
of delicate parenchymatous cells, c, corresponding to cambium 
cells, which are gradually converted into thick-walled prosenchy- 
matous cells, /, resembling those of the liber of Exogenous 
stems, together with some sieve-tubes ; and the whole bundle is 
surrounded by parenchyma, p. In this case the development 
of the fibro-vascular bundles, hke those of Exogenous stems, is 
