no 



MORPHOLOGY OF TISSUES. 



bundle may be termed Procamhium'^. As it grows older, some of its rows of 

 cells change into permanent cells of definite form (vessels, bast-fibres, &c.); and 

 from these starting-points the transformation of the procambium- cells into per- 

 manent cells in the transverse section of the bundle advances until the cells have 

 all undergone this change; or an inner layer of the bundle remains capable of 



Fig 92.— Transverse section of a closed fibro-vascular bundle in the stem of maize ( X 550) ; tlie fibro-vascular bundle 

 consists of the xylem-portion^'-^, J, r,l, and the phloem-portion w t' z'; the thick-walled tissue on the outside is the bundle- 

 sheath belonging- to the fundamental tissue ; // the thin-walled parenchyma of the fundamental tissue ; a outer side, /inner 

 side (facing the axis of the stem) ; g g twi> large pitted vessels ; J spirally thickened vessel ; r isolated ring of an annular 

 vessel ; / air-containing cavity, caused by splitting resulting from growth ; v v the cambiform or latticed cellular tissue which 

 has last passed over into permanent tissue ; between it and the vessel s lie reticulately thickened vessels with bordered pits. 



further development, and is then called Camhiuin. In advanced age there are thus 

 bundles devoid of and bundles containing cambium; the former may be termed 

 closed^ the latter open'^. As soon as a procambium -bundle has become transformed 



^ Nageli calls the tissue of the young fibro-vascular bundles simply Cambium, and applies 

 the same term to the tissue, capable of further development, of the bundles whicli increase in thick- 

 ness, which nevertheless ought to be distinguished from them. Sanio terms the latter only 

 Cambium, a restriction which I adopt. (Sanio in Bot. Zeitg. 1863, p. 362.) 



^ This distinction was first made by Schleiden ; but he incorrectly ascribed to Dicotyledons in 

 general only open bundles ; his distinction of simultaneous and successive cannot be sustained ; all 

 bundles become differentiated successively in transverse section. Schleiden's simultaneous bundles 

 of the higher Cryptogams belong to the closed description. 



