The two outer whorls of a well-developed perfect flower (calyx and corolla) together form the _perianth_. Some flowers, however have only one whorl outside the anthers, representing both the calyx and corolla of the more highly organised flower. This one whorl, therefore, is the perianth, and its parts are not correctly termed either petals or sepals, since they represent both. A perfect flower is sometimes spoken of as _bisexual_, for it includes the two sexual organs of the plant--the ovary or female part, producing the ovules; and the stamens or male part, which is concerned in the impregnation or fertilisation of the ovules. Many plants produce only _unisexual_ (and therefore imperfect) flowers, which contain either no stamens or no pistil. If such possess stamens and no pistil, they are called _staminate_ or _male_ flowers; and if pistil and no stamens, _pistillate_ or _female_ flowers. These two kinds are sometimes borne on the same plant, when they are said to be _monoecious_; but often on separate plants (_dioecious_), as in some of the Nettleworts and the Willow Tree. Spikes of unisexual flowers, such as are common among our forest trees, are called _catkins_. THE FRUIT AND SEED [Illustration: DEHISCENT FRUITS 1. Pod. 2. Siliqua. 3. Silicula. 4. Follicles (cluster of three). 5. Capsule splitting longitudinally. 6. Capsule splitting transversely. 7. Capsule splitting by pores.] After the ovules have been impregnated by the pollen they develop into seeds, each of which consists of or contains an embryo plant; and, at the same time, the ovary itself enlarges, changing its character more or less, till it becomes a ripened _fruit_. Fruits vary very considerably in their general characters, but may be divided into two main groups--those that split when ripe (_dehiscent fruits_) and those which do not split (_indehiscent fruits_). The principal forms of dehiscent fruits are:-- 1. The _pod_ or _legume_, which splits into two valves, with placenta on one side. 2. The _siliqua_, a long, narrow fruit that splits into two valves which separate from a membrane with placenta on both sides. 3. The _silicula_, of the same nature as the siliqua, but about as broad as it is long. 4. The _follicle_, which splits on one side only, through the placenta. 5. All other fruits that split are termed _capsules_. Some of these split longitudinally, some transversely, and others by forming pores for the escape of the seeds. The chief kinds of indehiscent fruits are:-- 1. The _drupe_ or stone-fruit, which consists of a hard stone surrounded by a fleshy covering, as the plum and the cherry. 2. The _berry_, which is soft and fleshy, and contains several seeds, like the currant and the grape. 3. The _nut_ or _achene_--a fruit with hard and dry walls, as the filbert and the acorn. 4. The _samara_ or winged fruit, like that of the sycamore.