Fig.
9.3-2. Magnification of parsnip cortex with ducts.
The duct consists of a single layer of cells called the epithelium.
These surround an accumulation
space, the lumen. From this micrograph alone, we cannot be certain
that this is a long duct cut in transverse section or a relatively spherical
cavity cut near the middle. But the low magnification view (Fig. 9.3-1, the
previous figure) had three ducts like this; if they were all spherical, it would
be suspiciously good luck to catch them all in median section. But if they are
long ducts, then we would expect to find many sections like that one.
The lumen here might have formed by the lysis (degeneration) of a central cell; if so, this is a lysigenous cavity. Or it could be that the epithelium cells had been adjacent and have since pulled apart (with the breakdown of the middle lamella); that would make this a schizogenous cavity. From this mature stage, we cannot tell which it is; we would need a much younger stage to examine.
It is difficult to determine if epithelium cells are or are not secretory. If they had densely cytoplasmic contents, that would be circumstantial evidence that they are secretory. But the fact that they are vacuolate and have little cytoplasm is not strong evidence that they are not secretory. It is entirely possible that there had been a central set of cells that filled with secretory product, swelled and forced the surrounding ordinary, non-secretory cortex cells into this shape, then the lumen cells lysed. The epithelium cells could be nothing more than displaced, flattened cortex; even so, they are still considered as epithelium because they are lining a secretory cavity.