96 



MORPHOLOGY OF TISSUES. 



layer of phloem is found in addition on the axial side of the xylem, so that the bundle 

 possesses two phloem-layers, a peripheral, and an axial {e. g. in Cucurbitaceae and 

 Nicotiana). In the closed bundles there occur, among Dicotyledons, considerable 

 deviations from the typical position of the tissues ; among Monocotyledons these are 

 still more conspicuous, especially if the sheath of lignified prosenchyma, which often 

 occurs with them, is taken into account (see Fig. 8i). Among Ferns, Lycopodiaceae 



(with isolated bundles ^), and Rhizo- 

 carpeae, the xylem lies in the centre 

 of the transverse section, while the 

 phloem forms a soft succulent sheath 

 around it (Figs 67 and 83). 



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 true 

 bast, &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 dif- 

 ferent parts of its length, when this 

 is considerable. The bundles of true 

 roots are frequently (not always) 

 without any true bast ; the termina- 

 tions of the bundles which traverse 

 the stem of Phanerogams are found 

 in the leaves; there, as their thick- 

 ness decreases, they lose all the ele- 

 ments 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 succulent or of cambiform cells (Fig. 16, F, p. 21). 



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 innermost vessels and the outermost bast-cells) 

 are the longest, since they participate in the whole increase of length of the organ ; 

 the elements developed later, during the elongation, are shorter ; and those are 

 shortest of all which arise after the increase of length of the whole organ has been 



Fig. 83.— a fourth of the transverse section of one of the large fibro- 

 vascular bundles in the stem of Pteris aqtiilina, with a portion of the 

 surrounding parenchyma,/; 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- g the vessels 

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

 Fig. 29 (p. 27) ; sp wide lattice-cells, between them and the xylem lies, 

 in the winter, a layer of cells containing starch ; b bast-like cells, with 

 thick soft wall ; sg the bundle-sheath ; between b and sg is a layer of 

 cells containing starch. 



vessels in the 'centre ; they are completely surrounded by cambiform tissue ; in Campanula lattfoUa, 

 according to the same authority, the bundles of the inner circle behave in the same manner as in 

 Ricinus. (Cf. Bot. Zeitg p. 179, 1865.) 



* The bundle in the stem of Lycnpodimn chamiEcyparissii':, &c., is clearly a union of several 

 fibro-vascular bundles 



