Tubers are modified stems or roots that can store nutrients. White potatoes are actually stem tubers. Tubers help plants survive dry spells or winter months, and can even be used to carry out asexual reproduction. Another modified plant structure is called a bulb. One can see a bulb in a tulip plant or an onion. It is a modified stem, with thick fleshy leaves that have both nutrients, water and a bud for reproduction in the following season. When you peel an onion, the thick, fleshy, moist leaves are evident. A corm, found in gladiolas, crocuses and freesia is an underground stem that stores nutrients and water during winter and during harsh conditions. It is made of cells containing starch. Corms can be cut into sections for asexual reproduction of the parent plant.