Fig. 5.1-5. Transverse section of ragweed stem (Ambrosia). These cells resemble the fibers of flax (Fig. 5.1-4), but these are vessel elements instead, involved in water conduction rather than in providing elastic strength. In transverse sections, it can be difficult to be certain if a cell is a vessel element or a fiber; features to consider: 1) very large cells (more than 30mm in diameter) with open lumens are almost certainly vessels, fibers are almost never so wide. 2) Fibers almost always occur as masses of fibers – most fibers have neighbors that are other fibers, so the cells here – each surrounded by thin-walled parenchyma cells with protoplasts – do not fit this criterion for fibers. In wood, vessel elements are often also completely surrounded by sclerenchyma (other vessels and fibers) and then it can be difficult to distinguish between fibers and narrow vessels. 3) Fibers will have narrow pits that are usually not very abundant; it might be necessary to look at a number of sections before actually seeing a pit (the flax fibers in Fig. 5.1-4 have an unusually high density of pits). Vessel elements on the other hand have a high density of very wide pits that have a border on their inner aperture. Unfortunately, to see these bordered pits in transverse sections, it is necessary to have very good sections and a good microscope; see Fig. 5.1-6.