They have a solid starchy interior stem. Crocuses are an example of corms. Tubers are modified stems with starchy interiors but with no flat supporting plate or tunic. Both roots and shoots sprout form the same growth buds, called eyes. Potatoes are considered to be tubers.
Rhizomes are thick underground stems.
They grow in a horizontal direction, sprouting new sections as they grow. Irises are considered to be rhizomes. 0 to 2 inches deep Irises - surface Plant hardy bulbs, such as lilies in early fall. Tender bulbs, such as irises should be planted in the late spring. Water well throughout the fall until the ground freezes to stimulate good root growth.