I lO 
MORPHOLOGY OF TISSUES. 
bundle may be termed Procambium'^. 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); the fibro-vascular bundle 
consists of the xylem-portion g s, r, I, and the phloem-portion vvv; the thick-walled tissue on the outside is the bundle- 
sheath belonging to the fundamental tissue ; / p the thin-walled parench3'ma of the fundamental tissue ; a outer side, z inner 
side (facing the axis of the stem); two 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 Cambium. In advanced age there are thus 
bundles devoid of and bundles containing cambium; the former may be termed 
closed, the latter ope7i'^. As soon as a procambium-bundle has become transformed 
Nägeli calls the tissue of the young fibro-vascnlar bundles simply Cambium, and applies 
the same term to the tissue, capable of further development, of the bundles which 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 cU-sed description. 
