Compositae : The largest natural order of plants, containing over 12,000 described species of herbs and shrubs distributed all over the world. The flowers are generally numerous and sessile, forming a close head on the dilated top of the receptacle, and surrounded by an involucre of whorled bracts. Many varieties are common weeds, like the daisy, dandelion and thistle, while many are cultivated in gardens, such as the aster, marigold, etc.