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 Archichlamydeac 
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. 
