132 



LECTURE VIIT. 



certain forms of vessels, may occasionally be very large, however, in transverse 

 section. The phloem and xylem likewise agree in that, apart from rare excep- 

 tions and later transformations, they possess no intercellular spaces of any kind; 

 the cells of a bundle all fit together closely on every side ; and in this they differ 

 conspicuously from the parenchyma of the fundamental tissue, which is interrupted 

 by intercellular spaces. 



The xylem, as well as the phloem, consists of various forms of cells ; and we may 

 say, for the purposes of a rapid survey, that in each of these two groups, vascular, 



FIG. 137 Transverse section of a closed fibro-vascular bundle of the stem of Zea Mays (XSSo), It Consists of 



xylem (^, g, S, r, /) and phloem [v, v). The surrounding thick-walled tissue is the bundle sheath, belonging to the 

 fundamental tissue. j», / thin-walled parenchynia of the fundamental tissue; a outer side; i inner side (facing the 

 axis of the stem} ; f . ^ two large pitted vessels ; s spiral vessel ; r isolated ring of an annular vessel ; / air cavity 

 produced by rupture during growth. 



fibrous, and parenchymatous elements occur. The homologous, and especially the 

 vascular, elements of the one group, however, are different from those of the other 

 group. In the more highly developed fibro-vascular bundles of terrestrial plants, the 

 cell-walls in the xylem are either totally, or at any rate to a great extent, lignified ; 

 especially the walls of the vessels and woody-fibres. When the lignification in the 

 xylem is reduced to a minimum, as occurs in very succulent tubers, tuberous roots, 

 and aquatic plants, the lignification is not unfrequently confined to the walls of the 

 vessels. In the phloem, either no lignification of the cell-walls occurs at all, or it 



