Bulbs

Bulbs are basically food storage mechanisms to enable the plant to grow in sometimes difficult conditions when a dormant plant begins to grow.  They are shoots which have modified leaves or thickened leaf bases.  In some cases, this means that the bulb needs nothing except light to grow, not even soil!  Whilst some bulbs like (narcissus) naturalise by sprouting additional bulbs each year, some bulbs like tulips tend to perform less well each year.  All bulbs can be left in the ground all year round however.  Whilst they are generally seen as the first colour of spring, bulbs are available which grow all year round providing colour at all times of year.  Other examples of bulbs are alliums, narcissus, hippeastrum, lily and tulip.

Bulbs are often confused with corms and tubers on a regular basis but are lumped together generally for convienience.

Corms are also storage systems but they consist of a swollen plant stem to store food. They have a protective skin and have one or more internodes and at least one growing point.  They are made up of starch-containing parenchyma cells. Crocus, Gladiolus, Iris, and Montbretia are all corms.

Tubers on the other hand are parts of rhizome which have increased in size to form the storage area. This modified stem is generally found underground towards the end of a lateral branch.

Allium
Anemone
Arum
Begonia
Chionodoxa
Colchicums
Crocus
Cyclamen
Eranthis
Erythronium
Fritillaria
Galanthus
Gladiolus
Hyacinthus
Iris
Leucojum
Lilium
Muscari
Narcissus
Nerine
Ornithogalum
Puschkinia
Scilla
Sternbergia
Tigridia
Tulipa

Annuals 
Biennials
Perennials
Shrubs 
Trees