Inflorescence, << `ihn` flaw REHS uhns, >> is the flowering region of a plant. Most botanists use this term only for a group of flowers. The single flower of such plants as the tulip and wake-robin is not classed as an inflorescence. In addition, flowers that grow singly at the bases of normal leaves, such as those of the pimpernel and morning-glory, are not considered inflorescences. Flowers in an inflorescence may be associated with modified leaves called bracts. In some plants, such as the flowering dogwood and the poinsettia, the bracts are larger and more noticeable than the flowers.
The two main kinds of inflorescences are indeterminate and determinate. In the indeterminate inflorescence, buds are produced and flower from the bottom to the top of the stem. Thus, the lower flowers on the stem are older than the upper ones. In another type of indeterminate inflorescence, the flat-topped inflorescence, all the flowers are at about the same level. In this type, the outer flowers are older than the inner ones.
Indeterminate inflorescences include the raceme, corymb, umbel, head, panicle, and spike. In a raceme, each flower is on a short stalk called a pedicel. The lily of the valley and the snapdragon have racemes. Corymbs, such as on the bridal wreath, are flat-topped clusters of flowers. The outer flowers have longer pedicels than the inner flowers. The umbel also is flat-topped, and its pedicels are all attached at the same point. Plants with umbels include the carrot and the dill. Plants with heads, such as the thistle and sunflower, have many small nonpediceled flowers tightly packed on the expanded top of the stem. The panicle is like the raceme except the pedicels branch, as in oats and many grasses. Spikes are similar to racemes except the flowers lack pedicels.
In the determinate inflorescence, or cyme, the bud at the tip of the stem develops into a flower, stopping the growth of that stem. Additional flowers can develop only on side branches. The central flower of each inflorescence is the oldest.
A third kind of inflorescence is the thyrse. This is a mixed arrangement, with the main stem indeterminate and the side branches determinate. The lilac is an example of this kind of arrangement.
