Complete and perfect flowers are hermaphroditic. The male organ or the androcele and the female organ or the gynocele are situated in one and the same flower. The unisexual flowers are those where the stamens and the pistils are produced on separate flowers, as in the willow. Stamen and pistil may not only be produced on separate flowers, but the staminate and pistillate flowers themselves may be borne on different plants, as in hickory, hazel, or Indian corn. Such plants are called dioecious or of two households. When both kinds of flowers appear on the same individual, the plant is called monoecious or of one household. [Illustration: CUT XXXVII. _Stamen._ a, anther; f, filament. After Bergen.] [Illustration: CUT XXXVIII. _Pollen grains._ s, spots where the inner coat bursts. After Bergen.] The stamen, or the male sexual organ of the flower, consists of a hollow portion called the anther, which is borne on a stalk called the filament. Inside of the anther is found a powdery or pasty substance called the pollen. The shape of the anther and the way in which it opens depend largely upon the way in which the pollen is to be discharged and how it is carried from flower to flower. As a rule, the anther opens by the cells being split length-wise, or by little holes at the top. The pollen in many plants is a fine dry powder, in others it is somewhat sticky or pasty. The forms of the pollen grains are various. Each pollen-grain consists of a single cell and is covered by a thick outer and a thin inner wall or coat. At the outer coat there are spots at which the inner coat of the grain is finally to burst through the outer one, pushing its way out in the form of a slender thin-walled tube. The contents of the pollen is a thickish protoplasma, full of little opake particles, and usually containing grains of starch and little drops of oil. [Illustration: CUT XXXIX. _Pistil._ sti, stigma; sty, style; ov, ovule. After Bergen.] The pistil, or the female sexual organ of the flower, usually consists of a small hollow chamber called the ovary, which contains the ovules, and of a slender portion or stalk, called the style. At the top of the latter is found a ridge, knob, or point which is called the stigma. The stigma consists of cells loosely arranged over the surface. These cells secrete a moist substance, to which the pollen grains adhere when they come in contact with the stigma. Beneath these superficial cells, running down through the style, there are found long cells, with intermediate spaces, through which the pollen tube reaches the ovary. [Illustration: CUT XL. _Stigma of thorn-apple with pollen._ p, pollen forming into a pollen-tube; i, intermediate space. After Bergen.] The ovules are not borne indiscriminately by any part of the lining of the ovary. They grow in a line running along one side of the ovary, as may be seen in the pea pod. This ovule-bearing line is called the placenta.