Fig. 9.—Head of flowers (capitulum) of Scabiosa atropurpurea. The inflorescence is simple and indeterminate, and the expansion of the flowers centripetal, those at the circumference opening first.

There are two distinct types of inflorescence—one in which the flowers arise as lateral shoots from a primary axis, which goes on elongating, and the lateral shoots never exceed in their development the length of the Inflorescence. primary axis beyond their point of origin. The flowers are thus always axillary. Exceptions, such as in cruciferous plants, are due to the non-appearance of the bracts. In the other type the primary axis terminates in a single flower, but lateral axes are given off from the axils of the bracts, which again repeat the primary axis; the development of each lateral axis is stronger than that of the primary axis beyond its point of origin. The flowers produced in this inflorescence are thus terminal. The first kind of inflorescence is indeterminate, indefinite or axillary. Here the axis is either elongated, producing flower-buds as it grows, the lower expanding first (fig. 8), or it is shortened and depressed, and the outer flowers expand first (fig. 9). The expansion of the flowers is thus centripetal, that is, from base to apex, or from circumference to centre.

Fig. 10.—Plant of Ranunculus bulbosus, showing determinate inflorescence.

The second kind of inflorescence is determinate, definite or terminal. In this the axis is either elongated and ends in a solitary flower, which thus terminates the axis, and if other flowers are produced, they belong to secondary axes farther from the centre; or the axis is shortened and flattened, producing a number of separate floral axes, the central one expanding first, while the others are developed in succession farther from the centre. The expansion of the flowers is in this case centrifugal, that is, from apex to base, or from centre to circumference. It is illustrated in fig. 10, Ranunculus bulbosus; a′ is the primary axis swollen at the base in a bulb-like manner b, and with roots proceeding from it. From the leaves which are radical proceeds the axis ending in a solitary terminal flower f′. About the middle of this axis there is a leaf or bract, from which a secondary floral axis a″ is produced, ending in a single flower f″, less advanced than the flower f′. This secondary axis bears a leaf also, from which a tertiary floral axis a″′ is produced, bearing an unexpanded solitary flower f″′. From this tertiary axis a fourth is in progress of formation. Here f′ is the termination of the primary axis, and this flower expands first, while the other flowers are developed centrifugally on separate axes.

A third series of inflorescences, termed mixed, may be recognized. In them the primary axis has an arrangement belonging to the opposite type from that of the branches, or vice versa. According to the mode and degree of development of the lateral shoots and also of the bracts, various forms of both inflorescences result.

Amongst indefinite forms the simplest occurs when a lateral shoot produced in the axil of a large single foliage leaf of the plant ends in a single flower, the axis of the plant elongating beyond, as in Veronica hederifolia, Vinca minor and Lysimachia nemorum. The flower in this case is solitary, and the ordinary leaves become bracts by producing flower-buds in place of leaf-buds; their number, like that of the leaves of this main axis, is indefinite, varying with the vigour of the plant. Usually, however, the floral axis, arising from a more or less altered leaf or bract, instead of ending in a solitary flower, is prolonged, and bears numerous bracteoles, from which smaller peduncles are produced, and those again in their turn may be branched in a similar way. Thus the flowers are arranged in groups, and frequently very complicated forms of inflorescence result. When the primary peduncle or floral axis, as in fig. 8, is elongated, and gives off pedicels, ending in single flowers, a raceme is produced, as in currant, hyacinth and barberry. If the secondary floral axes give rise to tertiary ones, the raceme is branching, and forms a panicle, as in Yucca gloriosa. If in a raceme the lower flower-stalks are developed more strongly than the upper, and thus all the flowers are nearly on a level, a corymb is formed, which may be simple, as in fig. 11, where the primary axis a′ gives off secondary axes a″, a″, which end in single flowers; or branching, where the secondary axes again subdivide. If the pedicels are very short or wanting, so that the flowers are sessile, a spike is produced, as in Plantago and vervain (Verbena officinalis) (fig. 12). If the spike bears unisexual flowers, as in willow or hazel (fig. 13), it is an amentum or catkin, hence such trees are called amentiferous; at other times it becomes succulent, bearing numerous flowers, surrounded by a sheathing bract or spathe, and then it constitutes a spadix, which may be simple, as in Arum maculatum (fig. 14), or branching as in palms. A spike bearing female flowers only, and covered with scales, is a strobilus, as in the hop. In grasses there are usually numerous sessile flowers arranged in small spikes, called locustae or 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.

Fig. 11.—Corymb of Cerasus Mahaleb, terminating an abortive branch, at the base of which are modified leaves in the form of scales, e. a′, Primary axis; a″, secondary axes bearing flowers; b, bract in the axils of which the secondary axes arise.
Fig. 12.—Spike of Vervain (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. 13.—Amentum or catkin of Hazel (Corylus Avellana), consisting of an axis or rachis covered with bracts in the form of scales, 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.
(From Strasburger’s Lehrbuchder Botanik, by permission ofGustav Fischer.)
Fig. 14.—Spadix ofArum maculatum. (AfterWossidlo.) a, Femaleflowers; b, male flowers;c, hairs representingsterile flowers.Fig. 15.—Compound umbel of CommonDill (Anethum graveolens), havinga primary umbel a, and secondaryumbels b, without either involucre orinvolucel.

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. 15. From the primary floral 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 umbellules. This inflorescence is seen in hemlock and other allied plants, which are hence called umbelliferous. If there are numerous flowers on a flattened, convex or slightly concave receptacle, having either very short pedicels or none, a capitulum (head) is formed, as in dandelion, daisy and other composite plants (fig. 2), also in scabious (fig. 9) and teazel. In the American button-bush the heads are globular, in some species of teazel elliptical, while in scabious and in composite 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 inflorescence of Dorstenia, or to that of the fig (fig. 6), where the flowers are placed on the inner surface of the hollow receptacle, and are provided with bracteoles. This inflorescence has been called a hypanthodium.

Lastly, we have what are called compound indefinite inflorescences. In these forms the lateral shoots, developed centripetally upon the primary axis, bear numerous bracteoles, from which floral shoots arise which may have a centripetal arrangement 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 Umbelliferae (fig. 15), 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. In Eryngium the shortening of the pedicels changes an umbel into a capitulum.