In some flowers the carpels are kept entirely distinct, each one having its own ovary, style, and stigma. For example, in the buttercup there is a little mound in the center of the flower made up of numerous pistils, each consisting of a single carpel. It is evident, therefore, that a pistil may consist of one carpel or several carpels, and that in the latter case the carpels may be more or less completely united. The sure indication of a carpel is that each carpel bears its own ovules. In some flowers there is but a single carpel, as in peas and beans, whose pods have developed from a pistil consisting of a single carpel, as is indicated by the single lengthwise set of seeds. In some plants the flowers do not have all the four parts described above. In some cases the petals may be lacking, the one set of perianth parts represented being regarded as the calyx, although it may look like a corolla, as in the clematis or anemone. Such flowers are said to be _apetalous_, which means "without petals." In other cases both the calyx and corolla may be wanting, the flower consisting of only stamens and carpels. Such flowers are spoken of as _naked_. In other flowers the stamens may be lacking, and as the pistil is the only essential part present such flowers are said to be _pistillate_. It may be counted upon, however, that if there are pistillate flowers there are also corresponding _staminate_ flowers in which the pistils are lacking and the stamens present. In such cases both staminate and pistillate flowers may occur on the same plant, or they may occur on different plants, so that there may be not only staminate and pistillate flowers, but also staminate and pistillate plants. It also sometimes happens that staminate and pistillate flowers are also naked, so that in such cases the flower is represented by stamens alone, or even by a single stamen, or by carpels alone, or by a single carpel. It would be hard to imagine a more simple flower than one composed of a single stamen or a single carpel. Such flowers may be found in the willows. In this study of the lily it should be observed that the number three runs through all the parts of the flower. The flower formula may be expressed as follows: sepals 3, petals 3, stamens 3 plus 3, carpels 3. This number is established in many families related to the lilies, and is one of their characteristic features. In other groups of flowering plants a different number is established, the number five being the most common. For example, in the common wild geranium the flower formula is as follows: sepals 5, petals 5, stamens 5 plus 5, carpels 5. In still other flowers the number four is established.