Fig.
7.2-9. Longitudinal section of vessel element of
corn. This is a very large, wide vessel element in corn, much wider than the
cells that surround it. Consequently, if we could see it in transverse section,
rather than having just four or five adjacent cells, it would have ten or twelve
or more. We can see that in this longitudinal section: the four arrows point out
four rows of pits, each row separated by a thin strip of red secondary wall. Each row of pits is occupying a separate
“panel” of vessel element side wall, each panel being the contact face
between the element and a different neighboring cell. In transverse
section, this vessel element is not round, but rather is a polygon, each side of
the polygon being one of these panels. The panel marked by X is wider than the
others; it is wide enough that the pits in it do not need to be circular –
they can be slightly scalariform. On the other hand, in some places it is wide
enough that rather than one scalariform pit, there are two circular bordered
pits more or less side by side.