in these so-called "ray-flowers." In the third group, of which the dandelion (Fig. 125, _H_), chicory, lettuce, etc., are examples, all of the flowers have strap-shaped, split corollas, and contain both stamens and pistils. The families of the _Aggregatæ_ are the following: I. _Rubiaceæ_ of which _Houstonia_ (Fig. 124, _A_), _Galium_ (_E_), _Cephalanthus_ (button-bush), and _Mitchella_ (partridge-berry) are examples; II. _Caprifoliaceæ_, containing the honeysuckles (_Lonicera_) (Fig. 124, _I_), _Viburnum_ (_G_), snowberry (_Symphoricarpus_), and elder (_Sambucus_); III. _Valerianeæ_, represented by the common valerian (_Valeriana_) (Fig. 124, _L_); IV. _Dipsaceæ_, of which the teasel (_Dipsacus_) (Fig. 124, _P_), is the type, and also species of scabious (_Scabiosa_); V. _Compositæ_ to which the innumerable, so-called compound flowers, asters, golden-rods, daisies, sunflowers, etc. belong; VI. _Calycereæ_. [Illustration: FIG. 126.--_Aristolochiaceæ_. _A_, plant of wild ginger (_Asarum_), × ⅓. _B_, vertical section of the flower, × 1. _C_, diagram of the flower.] Besides the groups already mentioned, there are several families of dicotyledons whose affinities are very doubtful. They are largely parasitic, _e.g._ mistletoe; or water plants, as the horned pond-weed (_Ceratophyllum_). One family, the _Aristolochiaceæ_, represented by the curious "Dutchman's pipe" (_Aristolochia sipho_), a woody twiner with very large leaves, and the common wild ginger (_Asarum_) (Fig. 126), do not appear to be in any wise parasitic, but the structure of their curious flowers differs widely from any other group of plants. CHAPTER XX. FERTILIZATION OF FLOWERS. If we compare the flowers of different plants, we shall find almost infinite variety in structure, and this variation at first appears to follow no fixed laws; but as we study the matter more thoroughly, we find that these variations have a deep significance, and almost without exception have to do with the fertilization of the flower. In the simpler flowers, such as those of a grass, sedge, or rush among the monocotyledons, or an oak, hazel, or plantain, among dicotyledons, the flowers are extremely inconspicuous and often reduced to the simplest form. In such plants, the pollen is conveyed from the male flowers to the female by the wind, and to this end the former are usually placed above the latter so that these are dusted with the pollen whenever the plant is shaken by the wind. In these plants, the male flowers often outnumber the female enormously, and the pollen is produced in great quantities, and the stigmas are long and often feathery, so as to catch the pollen readily. This is very beautifully shown in many grasses.