282 MORPHOLOGY 



two groups, the first three having hypogynous flowers, and the other two 

 having epigynous flowers. The hypogynous alliances are as follows : 



4. Gentianales (6 families, 4200 species). The combination of 

 characters that distinguishes this alliance from the next is the uniformly 

 opposite leaves and twisted aestivation (petals in the bud appearing as 

 if twisted around each other). As the name suggests, the gentian family 

 (Gentianaceae) is the characteristic representative of the alliance. 

 The curious and highly specialized milkweeds (Asclepiadaceae) are also 

 included here, characterized not only by their milky juice, but chiefly 

 by their elaborately insect-pollinated flowers and pollinia (see p. 258). 



5. Tubiflorales (20 families, 14,600 species). This great alliance 

 represents the culmination of the hypogynous Sympetalae, with con- 

 spicuous tubular corollas. The flowers range from regularity, as in the 

 morning glories (Ipomoea) and polemoniums (Polemoniaceae), to irreg- 

 ularity, as in the mints (Labiatae) and figworts (Scrophulariaceae). 

 The type of irregularity in this case is called bilabiate, which means that 

 the corolla develops a two-lipped mouth. These Tubiflorales with 

 irregular flowers hold the same position in reference to insect pollination 

 among Sympetalae that the Leguminosae do among Archichlamydeae, 

 and the Orchidaceae among monocotyledons. 



6. Plantaginales (i family, 200 species). This small alliance, 

 comprising the single family of plantains (Plantaginaceae), is charac- 

 terized by flowers with a membranous corolla and the cyclic number 

 four. Its relationships are obscure, but it is probably a reduction group. 



The two following epigynous alliances represent the culmination of 

 Sympetalae, and therefore of angiosperms. 



7. Rubiales (5 families, 4800 species). The madders constitute 

 the characteristic family (Rubiaceae), and associated with it are the 

 honeysuckles (Caprifoliaceae). The cyclic number is prevailingly four, 

 and there is a strong tendency to aggregate the flowers in close clusters. 



8. Campanales (6 families, 14,500 species). This highest alli- 

 ance is dominated by the great family Compositae (sunflowers, asters, 

 goldenrods, dandelions, etc.), which is not only the highest, but the 

 greatest of angiosperm families, including at least 12,500 species. It 

 adds to its sympetaly epigny, a seedlike fruit (achene), a special de- 

 velopment of its sepals as pappus, a complex organization of flowers 

 into a head so compact as to simulate a single flower, and usually a 

 differentiation of the flowers of a head into those that are showy (the 

 peripheral ray flowers) and those that are fertile (disk flowers). 



