[Illustration: FIG. 125.--_Anisocarpous sympetalæ_ (_Aggregatæ_). Types of _Compositæ_. _A_, inflorescence of Canada thistle (_Cirsium_), × 1. _B_, vertical section of _A_. _r_, the receptacle or enlarged end of the stem, to which the separate flowers are attached. _C_, a single flower, × 2. _o_, the ovary. _p_, the "pappus" (calyx lobes). _an._ the united anthers. _D_, the upper part of the stamens and pistil, × 3: i, from a young flower; ii, from an older one. _an._ anthers. _gy._ pistil. _E_, ripe fruit, × 1. _F_, inflorescence of may-weed (_Maruta_). The central part (disc) is occupied by perfect tubular flowers (_G_), the flowers about the edge (rays) are sterile, with the corolla much enlarged and white, × 2. _G_, a single flower from the disc, × 3. _H_, inflorescence of dandelion (_Taraxacum_), the flowers all alike, with strap-shaped corollas, × 1. _I_, a single flower, × 2. _c_, the split, strap-shaped corolla. _J_, two ripe fruits, still attached to the receptacle (_r_). The pappus is raised on a long stalk, × 1. _K_, a single fruit, × 2.] The last and highest order of the _Sympetalæ_, and hence of the dicotyledons, is known as _Aggregatæ_, from the tendency to have the flowers densely crowded into a head, which not infrequently is closely surrounded by bracts so that the whole inflorescence resembles a single flower. There are six families, five of which have common representatives, but the last family (_Calycereæ_) has no members within our limits. The lower members of the order, _e.g._ various _Rubiaceæ_ (Fig. 124, _A_, _E_), have the flowers in loose inflorescences, but as we examine the higher families, the tendency for the flowers to become crowded becomes more and more evident, and in the highest of our native forms _Dipsaceæ_ (Fig. 124, _P_) and _Compositæ_ (Fig. 125) this is very marked indeed. In the latter family, which is by far the largest of all the angiosperms, including about ten thousand species, the differentiation is carried still further. Among our native _Compositæ_ there are three well-marked types. The first of these may be represented by the thistles (Fig. 125, _A_). The so-called flower of the thistle is in reality a close head of small, tubular flowers (Fig. 125, _C_), each perfect in all respects, having an inferior one-celled ovary, five stamens with the anthers united, and a five-parted corolla. The sepals (here called the "pappus") (_p_) have the form of fine hairs. These little flowers are attached to the enlarged upper end of the flower stalk (receptacle, _r_), and are surrounded by closely overlapping bracts or scale leaves which look like a calyx; the flowers, on superficial examination, appear as single petals. In other forms like the daisy and may-weed (Fig. 125, _F_), only the central flowers are perfect, and the edge of the inflorescence is composed of flowers whose corollas are split and flattened out, but the stamens and sometimes the pistils are wanting