(_a_) _Apetalæ_--Including families where the petals are never present, and in some there is even no calyx. Examples: walnut, hickory, willow, and oaks. (_b_) _Polypetalæ_--Petals present but separate, not forming a tubular or connected corolla. Examples: buttercup, rose, pea, apple, geranium. (_c_) _Gamopetalæ_--Petals united and forming some sort of a tubular or connected corolla. Examples: garden primrose, gentian, salvia, mint, snapdragon, and the daisy family. Any attempt to describe the families contained in these three divisions of the dicotyledons would take all the rest of this book and crowd out other things about plant life that must not be omitted. All that can be done here is to outline briefly a few families in each division so that we shall have fixed in our minds what the general principles of plant classification are and how these are illustrated by well-known plants. There are many books that deal with this subject in great detail and to them the student should go for further elaboration of the subject. It is one of the most interesting phases of the study of botany, but it demands a longer and more intensive study than can be included here. (_a_) _Apetalæ_--These include families of plants that are the simplest in structure of all the dicotyledons. In all of them there are no petals and in some both petals and sepals are lacking, leaving only essential organs. Taking first those families that have neither petals nor sepals we find that most of them bear their flowers in catkins, a flower cluster familiar enough in the pussy willow. Some of these families are the following: _Juglandaceæ_--Trees with compound leaves, flowers in catkins and fruits inclosed in a thick husk. Examples: walnut and hickory. Six genera and over 30 species. _Salicaceæ_--Shrubs or trees with simple leaves, flowers in catkins and capsular many-seeded fruits, but no nuts. Containing only willows and poplars. Two genera and over 200 species. (Figure 93.) _Myricaceæ_--Shrubs or trees with simple usually fragrant leaves, flowers in catkins and fruits one-seeded. The bayberries. Two genera and about 35 species. (Figure 94.) [Illustration: FIG. 93.--PRAIRIE WILLOW (_Salix humilis_) The _Salicaceæ_, consisting only of willows and poplars, are always woody plants bearing their flowers in catkins.] These and some other families of close relationship are the least developed, in their flower structure, of any of the dicotyledons. All of them bear only the essential organs of reproduction in their very simple flowers. In the walnuts and hickories the different sexes are in different flowers on the same plant, in the willows they are even on different plants, and in Myricaceæ they are often found both ways. All of these flowers are wind-pollinated, so that they bear no honey, usually have no odor, and of course their need for showy petals for attracting insects is nil, although some pollen-eating insects visit them. [Illustration: FIG. 94.--SWEET FERN