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.
Fig. 451.
Head of sunflower showing centripetal inflorescence of tubular flowers. (Photo by the Author.)
828. Anthesis of flowers with indeterminate inflorescence.—In the anthesis of the raceme as well as in other corymbose forms the lower (or outer) flowers being older, open first. The opening of the flowers then takes place from below, upward; or from the outside, inward toward the center of inflorescence. The anthesis, i.e., the opening of the flowers of corymbose forms is said to be centripetal, i.e., it progresses from outside, inward. The anthesis of the fuller’s teazel is peculiar, since it shows both types. There are several distinct advantages to the plant where anthesis extends over a period of time, as it favors cross pollination, favors the formation of seed in case conditions should be unfavorable at one period of anthesis, distributes the drain on the plant for food, etc.
Fig. 452.
Heads of fuller’s teazel in different stages of flowering.