The youngest and largest plant family, comprising about seven hundred and fifty genera and ten thousand species, highly specialized for insect pollination, easily recognized as a whole, but many of its members difficult to distinguish. Seme tropical kinds are trees; ours are usually herbs, sometimes shrubs, without stipules; the leaves opposite, alternate or from the root; the flowers all small and crowded in heads, on the enlarged top of the flower-stalk, which is called the "receptacle," and surrounded by a common involucre of separate bracts, few or many, arranged in one or more rows; the receptacle also sometimes having scale-like or bristle-like bracts among the flowers, its surface smooth, or variously pitted and honey-combed. The flowers are sometimes perfect, or with only pistils, cr only stamens, or with stamens and pistils on different plants, or all kinds mixed. The calyx-tube is sometimes a mere ring, or its margin consists of hairs, bristles or scales, called the "pappus." The corollas are chiefly of two sorts; they are tubular and usually have five lobes or teeth, but often the flowers around the margin of the head are strap-shaped, that is, the border of the corolla is expanded into what is called a " ray." For instance, the yellow center, or "disk," of a Daisy is composed of a crowded mass of tiny tube-shaped flowers, which is surrounded by a circle of white, strap-shaped flowers, or rays, which look like petals. A Thistle, on the other hand, has no rays and the head is made up of tube-shaped flowers only. Stamens usually five, on the corolla-tube, alternate with its lobes, anthers usually united into a tube surrounding the style, which has two branches in fertile flowers, but usually undivided in sterile flowers; ovary inferior, one-celled, maturing into an akene, often tipped with hairs from the pappus to waft it about, or with hooks or barbs to catch in fur of animals. (Descriptions of genera have been omitted as too technical.)
There are many kinds of Carduus (Cnicus) (Cirsium), widely distributed; with tubular flowers only.