Fig. 7.1-5. Transverse section of vascular bundle in a bean seed (Phaseolus vulgaris). This section passes through one of the cotyledons of a bean (the cotyledons are the two large halves of beans, peas, and peanuts for example). Four arrows indicate four tracheary elements; notice that each is narrower -- much narrower -- than a single starch grain. Considering that starch grains are subcellular organelles, each of these tracheary elements is narrower than a small part of an ordinary parenchyma cell.

            Are these tracheary elements tracheids or vessels? Can't tell? Neither can I. With a longitudinal section and an excellent high power lens, we might just be able to see perforations if they are present and oriented properly. But even a tiny fragment of starch could hide the miniscule perforations these might have. And even if we could see a perforation, it might appear to be nothing more than a blemish, an artifact. We might look right at a perforation and still not be certain if these are vessel elements or tracheids.