The flower of a plant is a group of organs (figure 44) which exist for the purpose of producing seed. The parts of a flower fall into two general groups: those which actually function as seed producers (essential organs), and those which act as protective organs or organs for the attraction of insects (floral envelops or perianth). These might also be designated as nonessential organs, since they are lacking in certain flowers.
The essential organs consist of two parts, the pistils (figure 44A) and the stamens (figure 44D, E), often designated as the pistillate and stami-nate parts of a flower; and when a flower contains only pistils it is called a pistillate flower, and when it contains only stamens it is called a staminate flower.
When both stamens and pistils are present in the same flower, it is said to be perfect. If, in addition, the flower possesses the floral envelops, calyx and corolla, it is called a complete flower. Hence a flower which lacks any of these sets of organs is incomplete (that is, if it lacks either calyx or corolla); if it lacks either stamens or pistils it is imperfect.
The perianth or floral envelopes:
The ideal flower contains two sets of floral envelops, the calyx (figure 44F) and the corolla. In some flowers the corolla is entirely or partly divided into a certain number of divisions, each of which is called a petal (figure 44G). They are usually but not always brightly colored. Subtending or beneath the corolla is the calyx, which is usually, but not always, green, and is likewise in many plants divided into a number of distinct parts or sepals. When an incomplete flower has but one set of floral envelopes, it is usually the petals (or corolla) which are lacking, and in such cases the calyx may be brightly colored and function as a corolla (a petaliferous calyx).
The essential organs:
The number and arrangement of stamens varies in different kinds of plants, but nearly always a stamen consists of a filament or stalk (figure 44E), which bears at its apex the anther (figure 44D), or pollen-bearing sac. The shape of the anther, and the manner by which it dehisces, or opens to emit the pollen, likewise varies in different groups of plants.
The pistil (figure 44A-C), or seed-bearing organ, consists of an ovary (figure 44A), stigma (figure 44C and style (figure 44B). The ovary is at the base of the pistil and contains the ovules or eggs, which after fertilization ripen into seeds. The ovary usually contains several or many ovules, but may contain as few as a single ovule. The stigma is that part of the pistil which acts as a receptive organ for pollen in the process of pollination. Its surface is usually moist and minutely granular and its position and shape are dependent upon the mode of pollination (insects or wind) made use of by the particular plant. The style connects the stigma and ovary. It may be long or short, slender or stout, or sometimes entirely lacking when the stigma is situated directly upon the ovary.
The ovary itself may contain one or several chambers or cells (figures 45-47), and very frequently the number of chambers in the ovary and the lobes or divisions of the stigma bear a direct relationship to the number of petals, sepals and stamens. The term carpel (or carpophyllum) is used to designate the seed-bearing leaf. A carpel may be a pistil of itself, or it may be a constituent of a more complex pistil. In either case, a carpel is the homologue of a leaf. The surface within the ovary to which the ovules are attached is called the placenta.
Simple pistils may be solitary, or several together on a common receptacle within the flower, as in the Buttercup. A compound pistil consists of two, three or more carpels united into one body.
The apex of the flowering stem, which supports the flower, is designated as the receptacle.
Arrangement of flowers:
Flowers are either solitary or clustered, but their arrangement varies in different kinds of plants, and may even vary to some extent in the same species. The arrangement or disposition of the flowers may be designated as the inflorescence. The following are the most frequent arrangements of flowers:
Spike (figure 48), in which the flowers are arranged along the flowering stem, and sessile (that is, without stalks) or with very short stalks (pedicels). When the flowering stem is naked (devoid of leaves) and rises directly from the root or crown of the plant, it is called a scape (figure 51).
Raceme (figure 49), in which the flowers are arranged along a flowering stem and each flower possesses a distinct stalk or pedicel. The lower pedicels may be somewhat longer than the upper ones.
Umbel (figure 52), when the flowers arise from the same point, which is usually the apex of the flowering stem or of a lateral flowering stalk, and radiate like the rays of an umbrella. If the radiating stalks of such an inflorescence bear smaller umbels at their tips, it is called a compound umbel.
Corymb |corymbose| (figure 54), when the branches of an inflorescence are of unequal length, but the lower or outer ones are longest so that they all form a flat-topped, or nearly flat-topped, cluster.
Cyme (figure 55), when the flowers each terminate an axis or stem arising successively from a new axis or stem.
A spadix is a spikelike inflorescence with a fleshy stalk and with sessile flowers; the floral leaf or bract which subtends it or surrounds it partially is called the spathe (Skunk Cabbage, Wild Calla).
A panicle, or compound raceme (figure 50), is formed by the arrangement of flowers along the plant stem, similar to a raceme, but each flower stem has two or more branches.