THE FIBRO-VASCULAR BUNDLES. 



113 



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 diiferent 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- 

 Wcilled 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 Coniferge only prosenchymatous cells with 

 bordered pits (together with xylem-rays) are subsequently produced, so long as 



Fig. 94. — A fourth of a transverse section of one of the lar^e fibro- 

 vascular bundles in the stem of Ptn-is aqicilina, with a portion of the 

 surrounding' parenchyma, 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 ; .e £■ the vessels 

 thickened in a scalariform manner, the structure of which is explained in 

 Fig^. 29 (p. 28) ; J/ 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 ; sff the bundle-sheath ; between b and sg- is a layer of cells contain- 

 ing starch. 



