shaped corollas, x 1. _I_, a single
flower, x 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, x 1. _K_, a single fruit, x 2.]

The last and highest order of the _Sympetalae_, and hence of the
dicotyledons, is known as _Aggregatae_, 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 (_Calycereae_) has no members
within our limits.

The lower members of the order, _e.g._ various _Rubiaceae_ (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
_Dipsaceae_ (Fig. 124, _P_) and _Compositae_ (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 _Compositae_
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
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 _Aggregatae_ are the following: I. _Rubiaceae_ of
which _Houstonia_ (Fig. 124, _A_), _Galium_ (