Head (figure 53), an arrangement of flowers compactly on a common receptacle and surrounded by bracts (involucral bracts).
Modification and arrangement of the perianth:
Among the simpler groups of flowering plants the perianth is wanting, as in the Cat-tail and Willow. In the Sweet Flag, Oak and others, the perianth consists of a few scales, but in the higher plants, the perianth appears as a conspicuous portion of the flower, as in the Lily. Finally, as in the Rose family, there appears a clearly differentiated calyx and corolla.
In the simpler types of flowers, the sepals, petals and the stamens arise at the top of the receptacle. Such flowers are called hypogynous, meaning the insertion of these parts below the ovary (figure 56).
When the basal portion of the receptacle is continued upward, forms a cup-shaped growth around the ovary and bears the sepals, petals and stamens upon its margin, the flower is called perigynous (figures 57 and 59), meaning the insertion of the parts of the flower around the ovary.
Frequently the growth of the receptacle adheres to the ovary, and the sepals, petals and stamens appear to arise from above the position of the ovary, in which case the flower is called epigynous (figure 58), meaning above the ovary.
There may be varying degrees of cohesion or union of the parts of one or both of the floral envelopes (perianth). When the sepals are united with each other the Calyx is said to be gamosepalous, while a gamopetalous corolla (figures 62, 63, 64 and 65) refers to a union of the petals, as in the flower of the Morning-glory.
The degree of coalescence or union of parts of a gamopetalous corolla, varies in different flowers. When the calyx or corolla is divided almost to the base it is said to be parted (figure 63); when divided to about the middle it is said to be cleft (figure 64); when still less separated it may be said to be lobed or toothed (figure 65); or if entire on the margin it is said to be entire (figure 62).
When the parts of each set of organs of a flower are alike or equal in size, the flower is said to be regular, which means that the petals are alike, the sepals are alike and the stamens are alike. A symmetrical flower is one in which the sepals, petals and stamens are of the same number; unsymmetrical when there are unequal numbers in each cycle, that is, an unequal number of sepals, petals or stamens.
Certain groups of plants may often be recognized by the form of the corolla of some of its members. This character seems to be quite constant and the names of several large or important families of flowering plants are derived from this source. Of these groups we may mention the cruciferous (figure 60) type of flower of the Mustard family (Cruciferae), in which there are four spreading petals forming a cross, as in the flower of the Spring Cress (Cardamine bulbosa); the labiate corolla (figure 66) of the Mint family (Labiatae) in which the corolla is more or less two-lipped; the papilionaceous type of flower (figure 61) of the Pea family (Leguminosae), in which the petals are characteristically grouped into two lateral (wing) petals, a single upper (banner) petal and a pair of lower petals, often more or less united to form the keel.
The general characteristics of the stamens have already been described. In the stamens, as in the case of the petals and sepals, the number and arrangement are subject to great variation in different kinds of plants. Monandrous refers to a flower with a single or solitary stamen; polyandrous to a flower containing several stamens. The stamens may be monodelphous, in which the filaments are united into a tube, as in the Wild Lupine (figure 67), or the stamens may be diadelphous (figure 68), which means two sets of united stamens. In this form of arrangement there may be a union of the filaments of all the stamens except one, which is a common diadelphous arrangement of stamens in many of the species of the Bean family. When there are several sets of united stamens, the arrangement is said to be polydelphous.
Adnation or union of the stamens with other parts of the flower is of frequent occurrence, and the terms employed depend upon the degree of adnation, or the absence of it, namely, hypogynous (meaning beneath the pistil), applied to parts, including stamens, which are inserted or borne on the receptacle of the flower (figure 56). This is the absence of adnation and indicates an unmodified type. Perigynous (around the pistil) implies an adnation which carries up the apparent origin or place of insertion of the parts of the flower to some distance above or away from the receptacle and thus placing the insertion around instead of beneath the pistil (figure 57). Epigynous (on the pistil), where the adnation is complete to the very top of the ovary (figure 58).
When the stamens are borne upon the corolla, or upon the tube of the corolla, they are said to be epipetalous (figure 69), and when they are borne upon the pistil, as in the Orchid family, they are said to be gynandrous.
The most important part of a stamen is the anther (figure 44D), which contains the pollen. It normally consists of two lobes or sacs; but as each sac is often, and in most of our common flowers, divided into two cavities, it appears to possess in such instances four pollen sacs. For the discharge of the pollen, the cells of a normal anther open along a definite line, usually extending from top to bottom. This suture or line of dehiscence may be lateral or marginal, or centrally located.
In the genus Solanum, to which the Potato belongs, in most members of the Heath family (Ericaceae), in Polygala, and certain other species, the anther cells open only by a hole or pore (figure 71). In the Blueberry, Cranberry etc. the pore-bearing tip of the anther cell is prolonged considerably, often into a slender tube. In the Barberry, and in most other members of that family, and in the Lauraceae, the whole face of each anther cell separates by a continuous line, forming a kind of door, which is attached at the top, and turns back, as if on a hinge; and the anther is said to open by uplifted valves (figure 72). In the Sassafras and certain other members of the Lauraceae, each lobe of the anther opens by two smaller valves, like trapdoors.
The attachment of the anther to the filament (or stalk) presents three different modes, frequently connected by gradation: Innate (figure 70), in which the anther is a direct continuation of the axis of the filament, the cells usually opening by marginal slits, and the lobes or cells of the anther project neither inward nor outward; adnate (figure 73), in which the anther is a direct continuation of the filament but having the anther cells adherent to the anterior or posterior face of the filament; the Wild Ginger (Asarum) furnishes a good example of this, on account of a prominent prolongation of the connective or tip of the filament (figure 74); versatile (figure 75), when the anther is attached at some part only of its back or front to the tip of the filament, on which it lightly swings when the pollen is discharged; examples of this are seen in members of the Lily family, the grasses, Evening Primrose (Oenothera biennis) and others.