Two different modes of inflorescence are usually recognized in the arrangement of flowers on the stem. (1) The corymbose, or indeterminate inflorescence (also indefinite inflorescence), in which the flowers arise from axillary buds, and the terminal bud may continue to grow. (2) The cymose or determinate inflorescence (also definite inflorescence) in which the flowers arise from terminal buds. This arrests the growth of the shoot in length.
There are several advantages to the plant in the different modes of inflorescence, chief among which is the massing of the flowers, thus increasing the chances for effective pollination.
A. FLOWER CLUSTERS WITH INDETERMINATE
INFLORESCENCE.
818. The simplest mode of indeterminate inflorescence is where the flowers arise in the axils of normal foliage leaves, while the terminal bud, as in the florist’s smilax, the bellwort, moneywort, apricot, etc., continues to grow. The flowers are solitary and axillary. In other cases which are far more numerous, the flowers are associated into more or less definite clusters in which are a number of recognizable types. The word type used in this sense, it should be understood, does not refer to an original structure which is the source of others. It merely refers to a mode of inflorescence which we attempt to recognize, and about which we group those forms which have a resemblance to one another. There are many forms of flower clusters which do not conform to any one of our recognized types, and are very puzzling. The evolution of the flower clusters has been natural, and we cannot make them all conform to an artificial classification. These types are named merely as a matter of convenience in the expression of our ideas. The types usually recognized are as follows:
819. The raceme.—The flower-shoot is more or less elongated, and the leaves are reduced to a minute size termed bracts, while the flowers on lateral axes are solitary in the axils of the bracts. The reduction in the size of the leaves and the somewhat limited growth of the shoot in length, makes the flowers more prominent, and brings them into closer relation than if they were formed in the axils of the leaves on the ordinary foliage shoot. The choke cherry, currant, pokeweed, sourwood, etc., are examples of a raceme (fig. 569). In most plants with the raceme type, while the inflorescence is indeterminate, and the uppermost flowers (those toward the end of the main shoot) are younger, still the period of flowering is somewhat restricted and the raceme stops growing. In a few plants, however, as in the common “shepherd’s-purse,” the raceme continues to grow throughout the summer, so that the lower flowers may have ripened their seed while the terminal portion of the raceme is still growing and producing new flowers. Compound racemes are formed when by branching of the flower-shoot there are several racemes in a cluster, as in the false Solomon’s seal (Smilacina racemosa).
820. The panicle.—The panicle is developed from the raceme type by the branching of the lateral flower-axes forming a loose open flower cluster, as in the oat.
821. The thyrsus is a compact panicle of pyramidal form, as in the lilac, horse-chestnut, etc.
822. The corymb.—The corymb shows likewise an easy transition from the raceme type, by the shortening of the main axis of inflorescence, and the lengthening of the lower, lateral flower peduncles so that the flower cluster is more or less flattened on top. This represents the simple corymb. A compound corymb is one in which some of the flower peduncles branch again forming secondary corymbs, as in the mountain-ash. It is like a panicle with the lower flower stalks elongated.
823. The umbel.—The umbel is developed from the raceme, or corymb. The main flower-shoot remains very short or undeveloped with several flowers on long peduncles arising close together around this shortened axis, in the form of a whorl or cluster. Examples are found in the milkweed, water pennywort (Hydrocotyle), the oxheart cherry, etc. A compound umbel is one in which the peduncles are branched, forming secondary umbels, as in the caraway, parsnip, carrot, etc.
824. The spike.—In the spike the main axis is long, and the solitary flowers in the axils of the bracts are usually sessile, and often very much crowded. The plaintain, mullein ([fig. 422]), etc., are examples. The spike is a raceme, only the flowers are sessile and crowded. In the grasses the flower cluster is branched, and the branchlets bearing a few flowers are spikelets.