The Sunflower family comprises the largest group of flowering plants, including in the flora of the whole world about one-tenth of the known species, or some 12,000 in number. They are chiefly herbs in our region, but in warmer parts of the world, shrubs and tree forms also occur. In New York, about one-fifth of all plants which have rather conspicuous flowers, and might in consequence be designated as wild flowers, belong to this family. In this Memoir, over 50 species, or nearly one-eighth of the total number of plants illustrated, belong to the Sunflower family, and the number would be larger if it were not deemed unnecessary to illustrate all of the many kinds of Goldenrods and Wild Asters.
The chief characteristics of the family, which will aid considerably in an understanding of the descriptions of the following species, is the crowding together of the true flowers into heads. These floral heads, commonly referred to as the " flower," namely, the Sunflower, the Daisy etc. are in reality made up of many small, individual flowers, in contrast to the single flower of the rose or violet. The head is surrounded by an imrolucre, composed of one to several series of bracts or scales, performing as a whole the function of a calyx for the entire flower head. The individual bracts are often leaflike in character.
Two kinds of flowers arc to be noted in the " heads " of certain Compositae. They are the regular five-toothed or five-lobed corolla, as seen in the different species of Eupatorium, and the irregular, strap-shaped or ligulate corolla, in the ray flowers of the Wild Asters. Sometimes both kinds are found in the same head, as in the common Daisy, and then the strap-shaped flowers around the margin are referred to as ray flowers, and the densely packed tubular flowers in the center of the head are called disk flowers. Very often the disk flowers are of one color and the ray flowers of a different color. In some cases we find heads composed entirely of disk flowers (Thistles).
The stamens are five in number, attached to the inside of the corolla tube and usually cohere by their anthers in a ring around the style, which is commonly two-cleft at the summit, that is, a two-parted stigma. The ray flowers are usually without stamens, when disk flowers are present, and sometimes some flowers (either disk or ray, as the case may be) are entirely neutral (without stamens or pistils). The ovary is one-celled, containing one ovule which ripens into a small, dry, one-celled, one-seeded, indehiscent fruit, known as an achene. This achene is admirably adapted for seed dissemination by means of the persistent pappus, which matures along with the fruit. In the case of Bidens, the pappus consists of two barbed processes projecting from the achene, which catch in the shaggy coats of animals and on the clothing of passing persons and is thus distributed wide distances. In other cases the pappus consists of hairlike tufts, as in the Thistle, which enable the seed to be carried great distances by the wind.
The Compositae represent the most highly developed family of flowering plants, in respect to floral structure. By massing the flowers in heads, there is a great economy of space and tissue gained for the plant, and also greater certainty of pollination for the individual flowers, as a visit from one insect may result in the pollination of from several to many flowers. The showiness gained by massing the small flowers together serves as an added attraction to insects.
The Compositae contain many cultivated plants, including ornamental species. The Asters, Chrysanthemums, Pyrethrams, Gaillardias, Helen-rums, Helianthus (Sunflowers), Rudbeckias (Coneflowers), Dahlias and many others are some of the common and highly ornamental species.