THE FIBRO-VASCULAR BUNDLES. 
Ill 
into a closed fibro-vascular bundle, all further growth ceases, as in Cryptogams, 
Monocotyledons, and some Dicotyledons. The open fibro-vascular bundle, on 
the other hand, continues to produce new layers of permanent tissue on both 
sides of its cambium, and thus the portion of the stem or' root concerned con- 
tinually increases in thickness, as occurs in woody Dicotyledons and Conifers ; 
the foliar organs, however, of these plants possess closed bundles, or, if they are 
open, the activity of their cambium soon ceases. 
The different forms of tissue of a dififerendated fibro-vascular bundle may be 
classified into two groups, which Nageli calls the Phloem- (Bast) and Xylem- (Wood) 
portion of the bundle. They are separated by the cambium, if there is any. In 
each of the two constituents of the bundle, the phloem and the xylem, three 
forms of tissue are especially to be distinguished : — (i) Vascular cell-unions (the 
wood-vessels of the xylem, the sieve-tubes of the phloem) ; (2) Prosenchymatous 
Tissue (the wood-fibres in the xylem, the bast-fibres in the phloem); and 
(3) Parenchymatous Tissue (the wood-parenchyma in the xylem, the bast-paren- 
chyma in the phloem). The phloem consists of succulent, generally thin-walled 
cells ; only the bast-cells, which are often absent, but very frequently massively 
developed, are usually greatly thickened (mostly however not lignified but flexible). 
The thin-walled succulent cells are either parenchymatous, or they are cambiform 
or latticed-cells, or finally sieve-tubes. The xylem-portion of the fibro-vascular 
bundle has mostly a strong tendency to thicken its cell-walls, which become hard 
and lignified; in vessels and wood-cells with bordered pits the contents disappear, 
and they henceforth contain air. Lignified parenchyma is also abundant, but in 
some cases no lignifying takes place ; the whole bundle is then soft and succulent, 
sometimes traversed only by single thinner strings of lignified vessels and wood-cells, 
as in the roots of the radish, tubers of the potato, &c. The elements of the 
fibro-vascular bundles, as far as they consist exclusively of procambium, are mainly 
prosenchymatous, or at least elongated in the direction of the axis of growth of the 
bundle. In open bundles there arise also in the cambium, with the increase of their 
thickness, radial rows and layers of horizontally extended cells, by which the later- 
formed xylem- and phloem-layers of the bundle become broken up in a fim-like 
manner. These horizontal elements mostly assume the character of parenchymatous 
cells, and may be generally designated as rays; within the xylem they are called 
Xylem-rays, within the phloem Phloem-rays. 
The position of the layers of phloeni and xylem in the transverse section of a 
bundle varies according to the class to which the plant belongs and the organ in 
which they are found ; in the open bundle of the stem of Dicotyledons and 
Conifers the former lie towards the circumference \ the xylem facing the axis of 
the organ; between the two lies the cambium-layer (Fig. 93). But a layer of 
phloem is somedmes found in addidon on the axial side of the xylem, so that 
the bundle possesses two phloem-layers, a peripheral, and an axial-layer, e.g. in 
Cucurbitace^e, Solanaceae, and Apocynacese. In the closed bundles of Dicotyledons 
there occur considerable deviadons from the typical position of the tissues ; among 
^ See, however, what is said in Book II., at the end of the section on Dicotyledons, on the 
formation of tissue in that class. 
