The idea sounds right at first. We imagine that the fruit of a plant must be the easiest part to digest, and that the deeper we go toward the stem, root, or bulb, the tougher, rougher, and less useful it becomes for the human body. There is a little truth in that, but only a little.
Fruit is often easier to digest because it usually contains more water, more natural sugar, and less dense fiber than tougher plant parts. A ripe melon, orange, or banana tends to move through the body with much less effort than a dense raw root or a handful of nuts and seeds. But that does not mean every seed is easy, or that every root is hard, or that fruit is always the most nutrient rich.
Seeds are actually a great example of why the theory falls apart. Many seeds are harder to digest, not easier. They are concentrated little survival packs filled with fat, protein, fiber, and protective compounds. Some people do great with them. Other people feel bloated or heavy after eating them, especially if they are not soaked, ground, sprouted, or chewed well. In other words, seeds may be powerful foods, but they are not always gentle foods.
Roots and bulbs also deserve more respect than they usually get in these oversimplified ideas about nutrition. Carrots, beets, sweet potatoes, onions, garlic, ginger, and turmeric all come from underground or close to it, and many of them are packed with nutrients and beneficial compounds. Some are hard to digest raw, but much easier once cooked. A roasted sweet potato is not some inferior food just because it grew under the soil. In many cases, it is deeply nourishing and very digestible.
The real issue is not whether a plant part is a fruit, a seed, a stem, or a root. The real issue is texture, fiber, water content, ripeness, starch structure, and preparation. The harder, drier, more fibrous, and less ripe something is, the more work digestion may require. The softer, wetter, riper, and more cooked it is, the easier it often becomes for the body to handle.
So no, it is not true that the parts above the ground are automatically easier to digest and the parts below the ground are automatically less nutritious. Nature is not organized around our convenience. Plants are built for survival, reproduction, and defense. Some fruits are easy on the stomach. Some seeds are hard to break down. Some roots are incredibly healing and nourishing. The body responds not just to where the food comes from on the plant, but to what the food is made of and what we do to it before we eat it.
A more honest way to say it would be this: ripe fruit is often among the easiest plant foods to digest, but many roots and bulbs can be just as nourishing, and many seeds can be much harder on the digestive system than people assume. That is a more accurate place to begin.