Why onion and potato are called modified stems?

Answer 1

Bulbs can be considered to be very short stems encased in thickened, fleshy bulb scales (which are modified leaves). As the drawing below shows, the two basic bulb types are layered and scaly: * LAYERED BULBSare composed of a series of fleshy scales that form concentric rings when the bulb is cut in cross-section. In the picture at the right, both the onion bulb on the left and the garlic bulb on the right are layered bulbs. Well, the onion bulb is easy to recognize as a layered bulb but the garlic bulb is tricky because it looks like a scaly bulb. The difference between a garlic bulb and a scaly bulb is explained in the following section. * Tubers such as those shown at the tip of some Nut Grass stolons above, as well as the ordinary potato shown at the right, are often thought of as roots. However, as we've just said, roots don't have buds, and that's exactly what you see sprouting on the potato, arising from the potato's "eyes." Tubers are actually swollen portions of underground stems (stolons) and, as we've seen, stems have nodes, and buds arise at nodes. One reason it's hard to think of the potato with its sprouting eyes as an underground stem is that no nodes are obvious. If you were a scientist able to watch the potato's cells divide and grow from the very beginning you'd see that in the very early stages of development the potato had recognizable nodes, and then you could watch the nodes develop slowly into the potato's eyes, and the eyes would have buds associated with them, just like a normal tree-branch node. Bulbs can be considered to be very short stems encased in thickened, fleshy bulb scales (which are modified leaves). As the drawing below shows, the two basic bulb types are layered and scaly: * LAYERED BULBS are composed of a series of fleshy scales that form concentric rings when the bulb is cut in cross-section. In the picture at the right, both the onion bulb on the left and the garlicbulb on the right are layered bulbs. Well, the onion bulb is easy to recognize as a layered bulb but the garlic bulb is tricky because it looks like a scaly bulb. The difference between a garlic bulb and a scaly bulb is explained in the following section. * Tubers such as those shown at the tip of some Nut Grass stolons above, as well as the ordinary potato shown at the right, are often thought of as roots. However, as we've just said, roots don't have buds, and that's exactly what you see sprouting on the potato, arising from the potato's "eyes." Tubers are actually swollen portions of underground stems (stolons) and, as we've seen, stems have nodes, and buds arise at nodes. One reason it's hard to think of the potato with its sprouting eyes as an underground stem is that no nodes are obvious. If you were a scientist able to watch the potato's cells divide and grow from the very beginning you'd see that in the very early stages of development the potato had recognizable nodes, and then you could watch the nodes develop slowly into the potato's eyes, and the eyes would have buds associated with them, just like a normal tree-branch node.

Answer 2

Onion and potato are called modified stems because they originate from the underground part of the plant's stem rather than from the seed. These underground stems undergo modifications to store food and water, which help the plant survive in harsh conditions. The onion bulb and potato tuber are examples of modified stems that serve as storage organs for the plant.