The portion of the stem on which the flowers are borne is the flower shoot or axis, or taken together with the flowers, it is known as the Flower Cluster.
804. The flower.—The flower is best understood by an examination, first of one of the types known as a “complete” flower, as in the buttercup, the spring-beauty, the blood-root, the apple, the rose, etc.
There are two sets of organs or members in the complete flower—(1) the floral envelope; (2) the essential or necessary members or organs.
The floral envelope when complete consists of—1st, an outer envelope, the calyx, made up of several leaf-like structures (sepals), very often possessing chlorophyll, which envelop all the other parts of the flower when in bud; 2d, an inner envelope, the corolla, also made up of several leaf-like parts (petals), usually bright colored and larger than the sepals. The outer and inner floral envelopes are usually in whorls (though in close spirals in many of the buttercup family, etc.), and for reasons discussed elsewhere ([Chapter XXXIV]) represent leaves. The essential or necessary members of the flower are also usually in whorls and likewise represent leaves, but only in rare cases is there any suggestion, either in their form or color, of a leaf relationship. These members are in two sets: (1) The outer, or andrœcium, consisting of a few or many parts (stamens); (2) the inner set, the gynœcium, consisting of a few or many parts (carpels).
805. Purpose of the flower.—While the ultimate purpose of all plants is the production of seed or its equivalent through which the plant gains distribution and perpetuation, the flower is the specialized part of the seed plant which utilizes the food and energies contributed by other members of the plant organization for the production of seed. In addition to this there are definite functions performed by the members of the flower, which come under the general head of plant work, or flower work.
806. The calyx, or the sepals.—These are chiefly protective, affording protection to the young stamens and carpels in the flower bud. Where the corolla is absent, sepals are usually present and then assume the function of the petals. In a few instances the calyx may possibly ultimately join in the formation of the fruit (examples: the butternut, walnut, hickory).
807. The corolla, or petals.—The petals are partly protective in the bud, but their chief function where well developed seems to be that of attracting insects, which through their visits to the flower aid in “pollination,” especially “cross pollination.”
808. The stamens.—The stamens (= microsporophylls) are flower organs for the production of pollen, or pollen-spores (= microspores). The stalk (not always present) is the filament, the anther is borne on the filament when the latter is present. The anther consists of the anther sacs or pollen sacs (microsporangium) containing the pollen-spores, and the connective, the sterile tissue lying between and supporting the anther sac. The stamens are usually separate, but sometimes they are united by their filaments, or by their anthers. When the pollen is ripe they open by slits or pores and the pollen is scattered; or in rarer cases the pollen mass (pollinium) is removed through the agency of insects ([see Insect pollination, Chap. XLIII]).
809. The pistil.—The pistil consists of the “ovary,” the style (not always present), and the stigma. These are well shown in a simple pistil, common examples of which are found in the buttercup, marsh marigold, the pea, bean, etc. The simple pistil is equivalent to a carpel (= macrosporophyll), while the compound pistil consists of two or several carpels joined, as in the toothwort, trillium, lily, etc. The ovary is the enlarged part which below is attached to the receptacle of the flower, and contains within the ovules. The style, when present, is a slender elongation of the upper end of the ovary. The stigma is supported on the end of the style when the latter is present. It is often on a capitate enlargement of the style or extends down one side, or when the style is absent it is usually seated directly on the upper end of the ovary. The stigmatic surface is glutinous or “sticky,” and serves to hold the pollen-spores when they come in contact with it.
The ovules are within the ovary and are arranged in different ways in different plants. The pollen grain (or better pollen-spore = microspore), after it has been transferred to the stigma, “germinates,” and the pollen tube grows down through the tissue of the stigma and style, or courses down the stylar canal until it reaches the ovule. Here it usually enters the ovule (macrosporangium) at the micropyle (in some of the ament-bearing plants it enters at the chalaza), and the sperm cells are emptied into the embryo sac in the interior of the ovule.