THE FIBRO- VASCULAR BUNDLES. 
closed; while the former may be either closed or open. (Vide infra under the 
Fibro-vascular system of roots.) 
Every one of its cell-forms may at one time or other be absent from a fibro- 
vascular bundle ; bundles may occur without wood-cells, without vessels (very rarely), 
without bast-fibres, &c. ; it is only the soft bast (the succulent thin-walled cells of the 
phloem) that is scarcely ever absent. All these variations may occur in the same 
fibro-vascular bundle in different parts of its length, when this is considerable. The 
terminations of the bundles which traverse the stem of Phanerogams are usually 
found in the leaves; there, as their thickness decreases, they lose all the elements 
of the xylem except one or two spiral vessels, and finally these also ; the extreme 
ends of these bundles which traverse 
the mesophyll of the leaves often 
consist only of long narrow thin- 
walled cambiform cells. 
If the fibro - vascular bundle is 
formed at the very earliest period 
within an organ which afterwards 
grows rapidly in length, then the 
elements which were formed before 
the increase in length (the inner- 
most vessels and the outermost bast- 
cells) are the longest, since they par- 
ticipate in the whole increase in 
length of the organ ; the elements 
developed during the elongation are 
shorter; and those are shortest of 
all which arise after the increase in 
length of the whole organ has been 
completed ; this occurs in particular 
with the open bundles of Dicotyle- 
dons and Conifers. 
The development of the elements 
of a bundle always begins at single 
points in the transverse section, 
and extends from them in different 
directions; and thus the permanent 
cells which arise successively acquire different mature forms. In the open bundles 
in the stem of Dicotyledons and Gymnosperms the development usually begins 
with the thickening of single bast-cells on the peripheral side of the bundle; 
somewhat later single spiral vessels (or annular vessels) arise next the pith; 
and while the development of the phloem proceeds centripetally — forming succes- 
sively and often alternately bast-fibres, sieve-tubes and parenchyma— annular or 
spiral vessels with reticulate thickenings (or both forms), and eventually vessels 
with bordered pits alternating with wood-fibres and wood-parenchyma, arise 
centrifugally in the xylem (Fig. 95). In Coniferae only prosenchymatous cells with 
bordered pits (together with xylem-rays) are subsequently produced, so long as 
I 
Fig. 94.— a fourth of a transverse section of one of the larK'e fibro- 
vascular bundles in the stem of Pteris aqtiilina, with a portion of the 
surrounding: parencliyma, P, this is filled with starch (in winter) ; s spiral 
vessel in the focus of the elliptical transverse section of the bundle, sur- 
rounded by thin-walled wood-cells containing- starch ; ^ g the vessels 
thickened in a scalariform manner, the structure of which is explained in 
Fig. 29 (p. 28) ; sp wide sieve-tubes ; between them and the xylem lies, in 
the winter, a layer of cells containing starch ; b bast-cells, with thick soft 
wall ; the bundle-sheath ; between b and sg is a layer of cells contain- 
ing starch. 
