CLASSIFICATION OF SYMPETALAE 257 



probably be impossible, but an alliance at best expresses only a 

 general evolutionary tendency more or less completely worked 

 out. 



Taking the alliance as a whole, it represents the culmination 

 of liypogynous Sympetalae, and this culmination is shown not 

 only in the conspicuous corolla but in highly developed zygo- 

 morphism. In fact, the Personales, with the Labiatae and 

 Scrophulariaceae as centers of aggregation, represent the great 

 zygomorphic group of the Sympetalae, as Leguminosae do 

 among the Archichlamydeae, and Orchidaceae among the Mono- 

 cotyledons. 



First in the alliance are the Convolvulaceae and Polemonia- 

 ceae on account of their actinomorphic flowers and several- 

 ovuled carpels, in these and other features being, together with 

 the Gentianales, the least modified of the tetracyclic families. 

 From Gentianales they are easily distinguished by their lack 

 of twisted aestivation and by their usually alternate leaves, and 

 also by their undoubted relation to the other families of Tu- 

 biflorales. 



A second natural alliance is that formed by the Hydrophyl- 

 laceae and Borraginaceae, which leads from the preceding alli- 

 ance through Hydrophyllaceae, with a generally unlobed ovary, 

 to the Borraginaceae with a much modified ovary. In the latter 

 family the two carpels are divided by a false partition, each 

 loculus contains a single ovule, and the ovary becomes so deeply 

 lobed as to resemble a group of four nutlets. Further modi- 

 fications of this peculiar fruit, familiar to taxonomists, make 

 it the most specialized and diversified structure of this large 

 family. 



A third natural alliance is that formed by the Yerbenaceae 

 and Labiatae, with about 3,700 species. It is joined to the 

 Convolvulaceae by the orientation of the ovule, and has fol- 

 lowed a developmental path parallel with that of the preceding 

 alliance in the evolution of the carpel structures. The lobing 

 of the ovary into four nutlet-like bodies in the Labiatae, how- 

 ever, is not accompanied by such detailed specialization as in 

 the Borraginaceae ; but the whole line is dominated by the 

 strong development of zyo-omorphy, reaching its culmination in 

 certain groups of the Labiatae. 



A fourth natural alliance, the greatest of all, includes the 



