Fagaceæ—The oaks, beech, and chestnut. All trees or shrubs with at least their staminate flowers in catkins. Fruit inclosed in a bur (chestnut and beech) or borne in a cup (the acorns of oaks). At least five genera and about 375 species, widely distributed.
The habit of bearing some or all their flowers in catkins which flower usually before the leaves appear, and of having such flowers wholly at the mercy of precarious winds, is, if not lost, at least much less frequent in the remaining families of the apetalæ. All the others, while still without petals, do have sepals and some of these are colored so that insect visitors are likely. There are too many of these families to be enumerated here, but two of the chief are:
Ulmaceæ—The elms and hackberry. Trees or shrubs with minute greenish or yellowish flowers crowded in small clusters or in spikes. Fruit a dry nut or one-seeded and winged; or in the hackberry a drupe, one of the first evidences of even a slightly fleshy fruit in the dicotyledons. About 13 genera and 140 species, widely distributed.
Polygonaceæ—The buckwheat, knotweeds, common dock, and many other genera. Sepals often colored white or pink so the flowers are sometimes at least insect-pollinated. Flowers small and crowded in various clusters, often in a spike. Fruit an achene, a dry fruit familiar enough in the buckwheat. About 40 genera and over 800 species, mostly herbs or vines here, but often trees in the tropics.
From here on plant families leave, with some exceptions, the greenish or otherwise inconspicuous flower color, and somewhere about here they begin to rely more upon insect fertilization for the perpetuation of their kind. None of those so far mentioned have any petals to their flowers, but in the pink family or Caryophyllaceæ we find the first evidences on any considerable scale of the presence of sepals and petals, the latter usually beautifully colored. Familiar representatives of this family are the pink, carnation, chickweed, corn cockle, and the stichwort. There are over 50 genera and 1,000 species, nearly all in temperate regions.
The apetalous families appear to show a development from catkin-bearing trees with the sexes separated, and with neither petals nor sepals, through the Polygonaceæ, with often colored sepals, and the beginnings of insect fertilization. In Caryophyllaceæ, the most highly developed of them all, there are, besides the sepals, often or usually petals, and the reliance on insect fertilization is nearly complete. There are many transitional stages which cannot be included here, but they show step by step the development of the apetalous families from perfectly naked reproductive organs to the next larger group, the polypetalæ, where the process of increasingly complex flower development will now be sketched.
(b) Polypetalæ—In this large group of plant families the petals are free and quite separate, but as if they had not yet lost all the characters of the apetalæ, some families show incompletely the general characteristics of their more stable neighbors. There are, for instance, no petals in many species of the buttercup family, none in the sweet-gum tree nor in the maples, and a few others. But in spite of occasional exceptions this large group of polypetalous families do usually bear separate petals and sepals, and are among the most important of all the plant families. As they number over a hundred and contain thousands of species, all that can be done here is to mention a few typical or important ones. Just as in the apetalæ the families in this large group appear to show definite stages in development from simpler to more complex forms. But the steps are harder to trace and what appears simple characters in some plants are very complex in others.
While all the families in this group have separate petals some of them show a tendency to have united sepals, a character of perhaps some advantage and certainly very common among the still more developed gamopetalæ. Some of the families that have separate sepals agree in having the stamens inserted below the ovary. Of these the following three families may serve as types.