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 tbe alliance as a whole, it represents the culmination 

 of bypogynous iSympetalae, and tins culmination is shown not 

 only in tbe conspicuous corolla but in highly developed zygo- 

 morpbism. In fact, tbe Personales, with tbe Labiatae and 

 Scrophulariaceae as centers of aggregation, represent tbe great 

 zygomorphie group of tbe Sympetalae, as Leguminosae do 

 among tbe Arcbicblamydeae, and Orcbidaceae among tbe Mono- 

 cotyledons. 



First in tbe alliance are tbe Convolvulaceae and Polemonia- 

 ceae on account of their actinomorpbic flowers and several- 

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

 tbe Gentianales, tbe least modified of tbe 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- 

 bitlorales. 



A second natural alliance is that formed by the Hydrophyl- 

 laceae and Borraginaceae, which leads from the preceding alli- 

 ance through Hydrophyllaeeae, with a generally unlobed ovary, 

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

 family tbe 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 tbe Yerbenaeeae 

 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 tbe evolution of tbe carpel structures. Tbe lobing 

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

 ever, is not accompanied by such detailed specialization as in 

 the Borraginaceae ; but tbe whole line is dominated by the 

 strong development of zygomorphy, reaching its culmination in 

 certain groups of the Labiatae. 



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



