To understand how flower bulbs grow, you have to know what a true bulb is. A bulb is an underground root structure that holds the complete life cycle of a plant. It is made of five parts: the bottom plate, fleshy scales, tunic, shoot and lateral buds. Plants with true bulbs include tulips, daffodils, crocuses and hyacinths. Lilies are also true bulbs, but they are missing the tunic and need to be handled very carefully. Other plants such as cannas have rhizomes, and still other plants have tubers or corms. While these underground root systems are almost like bulbs, they are not true bulbs.
Main Plant Growth
The bottom plate, which is located at the very bottom of the true bulb, is the point from which the fleshy roots grow. The roots go into the soil and absorb water and nutrients necessary for the bulb and plant to grow. The extra nutrients and water are stored in the fleshy scales, which makes up the bulk of the bulb. Inside the bulb is also the main shoot, which in the spring starts to grow from the bottom plate, up through the fleshy scales and out of the bulb's protective tunic layer to become the plant. The foliage on the plant absorbs sunlight and, through the process of photosynthesis, the bulb turns that sunlight into sugar (food). Food, nutrients and water are used to keep the plant growing and blooming throughout the growing season.
New Bulb Formation
Next to the main shoot on the inside of the bulb are lateral buds---also called offsets. They are basically separate plants growing to the side of the main shoot. These lateral buds will form their own bulbs and new plants. At first, they use the nutrients from the main bulb, but in just a couple of seasons, they will have their own roots, fleshy scales (covered with a tunic) and foliage to feed themselves. These new bulbs can be divided later from the original bulb, if desired.
The plant will continue to absorb the light, convert it into sugar and store it in the fleshy scales of the bulb until it is either pruned back or allowed to wilt and turn brown in the fall. During that time, the bulb gets bigger, and there are more lateral buds forming until cold weather arrives, at which point bulbs go dormant. They stop growing and don't need water or nutrients. In most climates, bulbs can safely be kept in the ground covered with some mulch; however, in extremely cold environments, the bulbs should be carefully dug up and stored in a cool, dry location until they can be planted again in the spring, which is a good time to separate the lateral buds that have successfully turned into separate bulbs.