Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/133

Rh FORMS OF INFLORESCENCE.] B T A N Y 123 but regard it as a single flower with the floral leaves placed at different heights. In Grasses there are usually numerous sessile flowers arranged in small spikes, called locustai or Fig. 16]. FIG. 159. Corymb of Cerasus Jfahateb, produced in the axil of a leaf which has fallen, and terminating an abortive branch, at the base of which are modified .caves in the form of scales, e. FIG. 1GO. Spike of Verbena officinalis, showing sessile flowers on a common rachis. The flowers at the lower part of the spike have passed into fruit, those towards the middle are in full bloom, and those at the top are only in bud. FIG. 161. Amentum or catkin of Hazel (Corylut Avellana), consisting of an axis or rachis covered with bracts in the form of scales (squamae), each of which covers a male flower, the stamens of which are seen projecting beyond the scale. The catkin falls off in a mass, separating from the branch by an articulation. spikelets, which are either set closely along a central axis, or produced on secondary axes formed by the branching of the central one to the latter form the term panicle is applied. If the primary axis, in place of being elongated, is contracted, it gives rise to other forms of indefinite inflorescence. When the axis is so shortened that the secondary axes arise from a common point, and spread out as radii of nearly equal length, each ending in a single flower, or dividing again in a similar radiating manner, an umbel is produced, as in fig. 162. From the primary floral Fig. 162. Compound umbel of Common Dill (Anethum graveolens), having a primary umbel a, and secondary umbels b, without either involucre or involucel. The petiole p of the leaf is sheathing, and has been denominated pericladium. axis a the secondary axes come off in a radiating or umbrella-like manner, and end in small umbels b, which are called partial umbels or umbelhdes, to distinguish them from the general umbel formed by the branching of the primary axis. This inflorescence is seen in Hemlock, and other allied plants, which are hence called Umbelliferous. If there arc numerous flowers on a flattened, convex, or slightly concave receptacle, having either very short pedicels or none &amp;gt; a capitulum (head), anthodium, or calathium, is formed, as in Daiidelion, Daisy, and other Composite plants (fig. 1 IG), also in Scabiosa (fig. 157) and Dipsacus. In the American Button-bush the heads are globular, in some species of Teazel, elliptical, while in Scabious, and in Coin- Fig. 163. Peduncle a of Dorstenia, with concave receptacle 6. posite plants, as Sunflower, Dandelion, Thistle, Centaury, and Marigold, they are somewhat hemispherical, with a flattened, slightly hollowed, or convex disk. If the margins of such a receptacle be developed upwards, the centre not developing, a concave receptacle is formed, which may partially or completely enclose a number of flowers that are generally unisexual. This gives rise to the peculiar in florescence of Dorstenia (fig. 1G3), or to that of the Fig (fig. 150), where the flowers are placed on the inner surface of the hollow re ceptacle, and are provided with bracteoles. This in florescence has been called Jtypanthodium. Lastly, we have what are called compound inde finite inflorescences. In these forms the lateral shoots, developed centri- petally upon the primary axis, bear numerous brac teoles, from which floral shoots arise which may have a centripetal arrange ment similar to that on the mother shoot, or it may be different. Thus we may have a group of racemes, arranged in a racemose manner on a common axis, forming a raceme of racemes or compound raceme, as in Astilbe. In the same way we may have compound umbels, as in Hemlock and most Utnbelliferas (fig. 162), a compound spike, as in Rye-grass, a compound spadix, as in some Palms, and a compound capitulum, as in the Hen-and-Chickens Daisy. Again, there may be a raceme of capitula, that is, a group of capitula disposed in a racemose manner, as in Petasites, a raceme of umbels, as in Ivy, and so on, all the forms of inflorescence being indefinite in disposition. The elongation of secondary flower-stalks sometimes alters the general character of the inflorescence, changing a spike into a raceme, a raceme into a corymb, a capitulum into an umbel, and so forth. The capitulum of flowers in some Compositoe, such as Hypochaeris radicata and Senecio vulgaris, by a similar change in the pedicels assumes the form of an umbel. Among Umbelliferie the umbels are sometimes supported on very long stalks, while the pedicels of the individual flowers are not lengthened. In Eryngium the shortening of the pedicels changes an umbel into a capitulum. The umbellate inflorescence of Pelargonium has been seen changed into a raceme. The simplest form of the definite type of inflorescence is Definite seen in Anemone nemorosa and in Gentianella (Gentiana florescen acaulis, fig. 164), where the axis terminates in a single flower, no other flowers being produced upon the plant. This is solitary terminal inflorescence. If other flowers were produced, they would arise as lateral shoots from the bracts below the first-formed flower. The general name of cyme is applied to the arrangement of a group of flowers in a definite inflorescence. A cymose inflorescence is an inflorescence where the primary floral axis before ter minating in a flower gives off one or more lateral unifloral axes which repeat the process, the development being only limited by the vigour of the plant. The floral axes are thus centrifugally developed. The cyme, according to its development, has been characterized as l&amp;gt;iparous or vniparous. In fig. 165 the biparous cyme is represented in the flowering branch of Erythrcea Centaurium. Here the primary axis a ends in a flower/, which has passed