[Illustration: A section of a flower, cut lengthwise. In the center find the pistil with the ovary containing a number of ovules. Around this organ notice a circle of stalked structures, the stamens; the knobs at the end contain pollen. The outer circles of parts are called the petals and sepals, as we go from the inside outward.] The Use and Structure of a Flower.--It is a matter of common knowledge that flowers form fruits and that fruits contain seeds. They are, then, very important parts of certain plants. Our field trip shows us that flowers are of various shapes, colors, and sizes. It will now be our problem first to learn to know the parts of a flower, and then find out how they are fitted to attract and receive insect visitors. The Floral Envelope.--In a flower the expanded portion of the flower stalk, which holds the parts of the flower, is called the _receptacle_. _The green leaflike parts covering the unopened flower are called the sepals._ Together they form the _calyx_. _The more brightly colored structures are the petals._ Together they form the _corolla_. The corolla is of importance, as we shall see later, in making the flower conspicuous. Frequently the petals or corolla have bright marks or dots which lead down to the base of the cup of the flower, where a sweet fluid called _nectar_ is made and secreted. It is principally this food substance, later made into honey by bees, that makes flowers attractive to insects. The Essential Organs.--A flower, however, could live without sepals or petals and still do the work for which it exists. Certain _essential organs_ of the flower are within the so-called floral envelope. They consist of the _stamens_ and _pistil_, the latter being in the center of the flower. The structures with the knobbed ends are called _stamens_. In a single stamen the boxlike part at the end is the _anther_; the stalk which holds the anther is called the _filament_. The anther is in reality a hollow box which produces a large number of little grains called _pollen_. Each pistil is composed of a rather stout base called the _ovary_, and a more or less lengthened portion rising from the ovary called the _style_. The upper end of the style, which in some cases is somewhat broadened, is called the _stigma_. The free end of the stigma usually secretes a sweet fluid in which grains of pollen from flowers of the same kind can grow.