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 differentiated 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 fan-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 phloem 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 sometimes found in addition on the axial side of the xylem, so that 

 the bundle possesses two phloem-layers, a peripheral, and an axial-layer, e.g. in 

 Cucurbitaceae, Solanaceae, and Apocynacese. In the closed bundles of Dicotyledons 

 there occur considerable deviations 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. 



