Fig.
9.3-4.
Magnification of ducts in wormwood. There are two ducts here, the one on
the left is old enough to have developed a small lumen and epithelium; the one on the right has not yet reached the stage
of having a lumen. The arrow points to an area where the cells might
be starting to pull apart – the black material between the cells might be the
first stages of the breakdown of the middle lamella (there are places in the
upper part of this micrograph where there are similar amounts of dark material
between cells, but those are the thickened corners of collenchyma cells). These
young ducts occurred in the same histological section as older ducts, indicating
that duct development is not completely simultaneous: it is fairly common in
many species for ducts to be initiated at slightly different times or for some
to mature more rapidly than others. However, it is somewhat unusual for a new
duct to be initiated in a portion of stem, leaf or root that is so old it has
completely mature, fully differentiated ducts already.