Tulips are an example of tunicate bulbs. These bulbs have outer scales that are dry and membranous. The covering that surrounds the bulb is known as the tunic. It provides protection from drying and mechanical injury. The fleshy scales are in continuous layers that produce a solid bulb structure.
Within the tunicate bulbs, there are 3 basic structures. The structural classification will depend on the type of scales and growth pattern of the bulb. The first type of bulb structure has expanded scales on the bases of the leaves, which is seen in Amaryllis bulbs. The second type has expanded leaf bases and true scales, like that in Narcissus bulbs. The last type has only true scales and these leaves are produced on a vegetative or flowering shoot, like in tulip bulbs.
In tunicate bulbs, adventitious root primordia are present in the dormant bulb. These roots only elongate under the proper conditions. The roots are found in the narrow band along the outside edge of the basal plate. In the tunicate bulbs, small bulblets are formed in the axils of the bulb scales at the leaf base.
Non-tunicate bulbs, or scaly bulbs, can be found in lilies. These bulbs do not have a dry covering surrounding them. The scales are instead separate and are attached to the basal plate. Generally, non-tunicate bulbs are easily damaged and must be handled more carefully than the tunicate bulbs. These bulbs also need to be kept continually moist.
In non-tunicate bulbs, stems develop in the midsummer and persist until the following year. Roots can also form on the stem above the bulb. Many species also possess thickened, contractile roots. These roots shorten and pull the bulb into a given level on the ground. Species like tulips do not have contractile roots. Instead, they produce droppers. These are stolon-like structures that grow from the bulb and produce new bulbs at their tips.
Check out our bulb playlist on Youtube for all you need to know!