=Conformation of the Flower.=--A flower that has calyx, corolla, stamens, and pistils is said to be =complete= (Fig. 173); all others are =incomplete=. In some flowers both the floral envelopes are wanting: such are =naked=. When one of the floral envelope series is wanting, the remaining series is said to be calyx, and the flower is therefore =apetalous= (without petals). The knotweed (Fig. 179), smartweed, buckwheat, elm are examples. [Illustration: FIG. 179.--KNOTWEED, a very common but inconspicuous plant along hard walks and roads. Two flowers, enlarged, are shown at the right. These flowers are very small and borne in the axils of the leaves.] Some flowers lack the pistils: these are =staminate=, whether the envelopes are missing or not. Others lack the stamens: these are =pistillate=. Others have neither stamens nor pistils: these are =sterile= (snowball and hydrangea). Those that have both stamens and pistils are =perfect=, whether or not the envelopes are missing. Those that lack either stamens or pistils are =imperfect= or =diclinous=. Staminate and pistillate flowers are imperfect or diclinous. [Illustration: FIG. 180.--STAMINATE CATKINS OF OAK. The pistillate flowers are in the leaf axils, and not shown in this picture.] [Illustration: FIG. 181.--BEGONIA FLOWERS. Staminate at _A_; pistillate below, with the winged ovary at _B_.] When staminate and pistillate flowers are borne on the same plant, _e.g._ oak (Fig. 180), corn, beech, chestnut, hazel, walnut, hickory, pine, begonia (Fig. 181), watermelon, gourd, pumpkin, the plant is =monœcious= (“in one house”). When they are on different plants, _e.g._ poplar, cottonwood, bois d’arc, willow (Fig. 182), the plant is =diœcious= (“in two houses”). Some varieties of strawberry, grape, and mulberry are partly diœcious. Is the rose either monœcious or diœcious? [Illustration: FIG. 182.--CATKINS OF A WILLOW. A staminate flower is shown at _s_, and a pistillate flower at _p_. The staminate and pistillate are on different plants.] [Illustration: FIG. 183.--FLOWER OF GARDEN NASTURTIUM. Separate petal at _a_. The calyx is produced into a spur.] [Illustration: FIG. 184.--THE FIVE PETALS OF THE PANSY, detached to show the form.] [Illustration: FIG. 185.--FLOWER OF CATNIP.] Flowers in which the parts of each series are alike are said to be =regular= (as in Figs. 173, 174, 175). Those in which some parts are unlike other parts of the same series are =irregular=. Their regularity may be in calyx, as in nasturtium (Fig. 183); in corolla (Figs. 184, 185); in the stamens (compare nasturtium, catnip, Fig. 185, sage); in the pistils. Irregularity is most frequent in the corolla.