=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. =825. The head.=—When the flower axis is very much shortened and the flowers crowded and sessile or nearly so, forming a globose or compressed cluster, it is a _head_ or _capitulum_. The transition is from a spike by the shortening of the main axis, as in the clover, button bush (_Cephalanthus_), etc., or in the shortening of the peduncles in an umbel, as in the daisy, dandelion, and other composite flowers. In these the head is surrounded by an involucre, which in the young head often envelopes the mass of flowers, thus affording them protection. In some other composites (Lactuca, for example) the involucre affords protection for a longer period, even while the seeds are ripening. =826. The spadix.=—When the main axis of the flower cluster is fleshy, the spike or head forms a _spadix_, as in the Indian turnip, the skunk-cabbage, the calla, etc. The spadix is usually more or less enclosed in a _spathe_, a somewhat strap-shaped leaf. =827. The catkin.=—A spike which is usually caducous, i.e., falls away after the maturity of the flower or fruit, is called a catkin, or an _ament_. The flower clusters of the alder, willow, (fig. 555), poplar, and the staminate flower clusters of the oak, hickory, hazel, birch, etc., are _aments_. So characteristic is this mode of inflorescence that the plants are called _amentiferous_, or _amentaceous_.