What point does the professor make about closely related plants such as wheat and barley?

Listen to part of a lecture in an archaeology class.
Professor: Archaeologists have many reasons to be interested in studying the remains of plants. Historically, humans have relied on plants as a source of everything, from building material to food and medicine. Naturally, identifying plant remains at ancient human sites would help us gain insight into the way of life of a culture at a given point in time, but plants decay quickly, so it’s rare to find intact specimens to examine. Even such seemingly sturdy stuff as seeds don’t hold up over time, so what do we do? Fortunately, some plants leave behind microscopic traces of themselves that don’t decay. Pollen and starch grains, for example, have been used in archaeological studies, as have what are called "phytoliths." Phytoliths are very tiny, mineral deposits, almost like pieces of hard glass, that form in the cells or between cells of many kinds of plants. The minerals originate in the ground water that the plant absorbs through its roots. When the water runs through the plant’s cells, the minerals are deposited. So a phytolith is essentially a cast, a 3-D impression of a plant’s cell or, depending on the species, of the space between cells, and when the plant dies, its phytoliths are released into the soil.
    The plant suffers no ill effects from the phytoliths. Actually, plants can benefit from them. For example, leaves dense with phytoliths are ridged and better able to absorb the sun’s rays for photosynthesis. And these leaves can also deter hungry herbivores. Now, think about it for a moment. If you were an herbivore, a plant eater, would you prefer to chew on a nice, soft leaf or one full of hard mineral deposits?
    Now, researchers have been actively analyzing phytoliths for about four decades, and they’ve proved to be extremely important. You see, in addition to the fact that phytoliths don’t decay, the shape of a species’ phytoliths is as unique as your fingerprints are to you, so we can tell what species left behind the phytoliths we happen to be studying, and with advances in technology, like computer imaging and the electron microscope, we can even distinguish between closely related plants like wheat and barley. That incidentally is something we couldn’t do until recently because the only tool we had was the standard light microscope. The electron microscope, which uses electron beams, allows us to see details we’d never see with light microscopes.
    Ok, so what exactly can phytoliths tell us about the past? Well, their presence in residue in vessels used for cooking and storing food lets us know what a population was eating. We can also determine when a particular plant was first domesticated. For example, we use phytoliths to distinguish between wild and domesticated maize. This is actually something researchers want to know about maize, when did people first start to plant it as a crop. So what we’ve been able to do is identify ancient sites of maize agriculture, take samples from different soil layers, and study the phytoliths from each layer. The deeper the layer, the older the soil is and we can date the soil layers. So far we’ve been able to trace domesticated maize back nearly 10,000 years. It could be even older, of course. Technology is making advances all the time. Now, we couldn’t have studied the domestication of maize without phytoliths. Granted pollen can be useful. As I said, it resists decay and it’s possible to determine to some degree what plant a grain of pollen came from, but consider the purpose of pollen. It’s to be carried away from the plant, right, by wind or animals. The plant contributes to the continuation of its species by spreading pollen around, I’d argue. Therefore, that pollen is only a very loose sort of indicator of land use for the site where it’s found. On the other hand, phytoliths are generally found very close to where the plant lived. They’re deposited in the soil where the plant decayed. So, if you find a high concentration of phytoliths from a domesticated species of plant in one area, it could be an indication that you may have discovered a field, an agricultural field. Now, there are several databases and guides that identify the phytoliths of thousands of species. This means when we come across a phytolith, we can look it up in these resources and find out what plant it came from.