Bulbs: Bulbs are specialized underground stem. In bulb stem is reduced and become discoid. On the lower side of the disc adventitious roots develops in cluster and the upper surface show compactly arranges fleshy leaf bases or scale leaf the leaf base become fleshy due to accumulation of carbohydrates. Bulbs are of two types tunicated and imbricated.
Tunicated: Fleshy leaf bases arranged in concentric manner and the entire leaf is covered by peripheral dry membranous leaf bases called tunics Example: garlic
Imbricated: Fleshy scale leaf arranged loosely overlapping one another such bulbs are not covered by any tunics Example: onion
Corms: Underground modification of stem grows vertical. They can be characterized by circular nodes. Terminal bud is large. Corm bears scale leaves at each node. With the help of some special adventitious roots called as contractile roots, the corm remains constantly at a particular depth.
Tubers: Grow underground at any depth. The axillary branches that are produce in the soil and their tip become swollen due to accumulation of starch n proteins. Eyes are found on the axillary bud and covered with scaly leaves Example: potato.