The function of the flower is to _produce seed_. It is probable that all its varied forms and colours contribute to this supreme end. These forms and colours please the human fancy and add to the joy of living, but the flower exists for the good of the plant, not for the good of man. The parts of the flower are of two general kinds—those that are directly concerned in the _production of seeds_, and those that act as _covering and protecting organs_. The former parts are known as the =essential organs=; the latter as the =floral envelopes=. [Illustration: FIG. 173.—FLOWER OF A BUTTERCUP IN SECTION.] =Envelopes.=—The floral envelopes usually bear a close resemblance to leaves. These envelopes are very commonly of two series or kinds—the _outer_ and the _inner_. The outer series, known as the =calyx=, is usually smaller and green. It usually comprises the outer cover of the flower-bud. The calyx is the lowest whorl in Fig. 173. The inner series, known as the =corolla=, is usually coloured and more special or irregular in shape than the calyx. It is the showy part of the flower, as a rule. The corolla is the second or large whorl in Fig. 173. The _calyx_ may be composed of several leaves. Each leaf is a =sepal=. If it is of one piece, it may be lobed or divided, in which case the divisions are called =calyx-lobes=. In like manner, the corolla may be composed of =petals=, or it may be of one piece and variously lobed. A calyx of one piece, no matter how deeply lobed, is =gamosepalous=. A corolla of one piece is =gamopetalous=. When these series are of separate pieces, as in Fig. 173, the flower is said to be =polysepalous= and =polypetalous=. Sometimes both series are of separate parts, and sometimes only one of them is so formed. [Illustration: FIG. 174.—FLOWER OF FUCHSIA IN SECTION.] _The floral envelopes are homologous with leaves._ Sepals and petals, at least when more than three or five, are in more than one whorl, and one whorl stands below another so that the parts overlap. They are borne on the expanded or thickened end of the flower stalk; this end is the =torus=. In Fig. 173 all the parts are seen as attached to the torus. This part is sometimes called the _receptacle_, but this word is a common-language term of several meanings, whereas torus has no other meaning. Sometimes one part is attached to another part, as in the fuchsia (Fig. 174), in which the petals are borne on the calyx-tube. =Subtending Parts.=—Sometimes there are _leaf-like parts just below the calyx_, looking like a second calyx. Such parts accompany the carnation flower. These parts are =bracts= (bracts are small specialized leaves); and they form an =involucre=. We must be careful that we do not mistake them for true flower-parts. Sometimes the bracts are large and petal-like, as in the great white blooms of the flowering dogwood: here the real flowers are several, small and greenish, forming a small cluster in the centre.