472 ANGIOSPERMS 



Upon differences which pertain chiefly to the flowers, the 

 Monocotyledons and Dicotyledons are subdivided into many 

 groups. 



The Monocotyledons are subdivided into 8 or 10 orders which 

 are in turn subdivided into about 42 families. The families are 

 subdivided into many genera and the genera into species of 

 which there are about 25,000. 



The Dicotyledons, of which there are more than 100,000 spe- 

 cies, include most of the Angiosperms, being more than four 

 times as numerous as the Monocotyledons. The Dicotyledons 

 are divided into two large subdivisions — the Archichlamydeae 

 and the Sympetalae. 



The Archichlamydeae have a corolla of separate petals or no 

 corolla at all. They include about 180 families and 61,000 

 species of Dicotyledons. They are grouped into two classes, 

 one of which has apetalous flowers, that is, flowers without 

 petals, and the other of which has polypetalous flowers, that is, 

 flowers with petals present and free from each other. 



The Sympetalae include those Dicotyledons in which the 

 petals are more or less united. There are about 50 families 

 and 42,000 species of the Sympetalae. 



In arranging the orders and families taxonomists have en- 

 deavored to follow an evolutionary sequence. The rank of an 

 order or family depends chiefly upon the organization of its 

 flowers. Flowers most like a typical strobilus, that is, resem- 

 bling most the strobili of Gymnosperms, are regarded as the 

 simplest of flowers. Thus a flower without any perianth is 

 simpler than one with a perianth. Also a flower with parts 

 arranged spirally, thus having parts arranged like the sporophylls 

 in a strobilus, is considered simpler than one with parts having a 

 cyclic arrangement. Again flowers having petals joined or carpels 

 united are considered more advanced than flowers in which these 

 parts are separate. Thus the Sympetalae, since they have united 

 petals, are considered more advanced than the Archichlamydeae 

 which have separate petals or no petals at all. In respect to 

 these evolutionary tendencies the orders and families of both 

 Dicotyledons and Monocotyledons form an ascending series. 



Most of the families of the Angiosperms have some species 

 of economic importance, but some families are much more 

 notable than others for their species related to man's welfare. 



