Rhizome or Bulb?
This question is from Celia in Little Rock, and it is a very pertinent question, since many people and a lot of home gardeners indeed very often use the terms interchangeably, and with some justification for that use.
Certainly members of a specialty plant society such as an iris society dedicated to the growing of iris, or members of a daffodil society who are equally dedicated to their favorite plant, all with some years of seniority in their respective fields, would not use the term in this loose broad general manner. Experienced gardeners would instinctively know and appreciate the difference between the terms, as well as the inherent nature of the underlying structures to which they refer, that is in particular the make up of a rhizome and a bulb.
A Daffodil grower of experience would no more refer to the underground structure that produces his or her favorite plant as a rhizome, as an iris grower would call a rhizome a bulb. The term 'bulb' has been used for centuries to refer to all the under ground storage structures that plants use to store nutrients for future use. Most reference sources use the term to denote a class of plants that produce underground storage structures. When used to denote a class of plants it should not be considered an incorrect usage of the term.
Merriam-Webster Collegiate Dictionary has it right, it defines the term first as: " (a) a resting stage of a plant (as the lily, onion, hyacinth, or tulip) that is usually formed underground and consists of a short stem base bearing one or more buds enclosed in overlapping membranous or fleshy leaves (b) a fleshy structure (as a tuber or corm) resembling a bulb in appearance (c) a plant having or developing from a bulb".
To me a bulb contains the complete life cycle of a plant with in itself. Amaryllis and Paper white narcissus demonstrate this succinctly by growing vigorously from stored reserves when being anchored in a non-nutrient containing medium and supplied moisture only. They bloom quite vigorously from the nutrients stored with in the bulb from the preceding year's growth cycle. The nutrients were stored during the primary growing season, which mostly occurs after the bloom of the previous year. Bulbs forced in this way are truly spent after being forced to bloom without additional nutrients being supplied during the forcing period, and they should be discarded and not planted in the garden.