456 STRUCTURE AND LIFE HISTORIES 
in dicotyledons the parts are typically in fours or Jives. 
These characters are illustrated diagrammatically in Figs. 
338 and 339. 
405. Groups of Dicotyledons. There are two main 
groups of dicotyledons, based on the fusion or non-fusion 
of the parts of the calyx or corolla, as follows: 
/ Apetalae 
~. , , Archichlamydeae < _ . . , 
Dicotyledons < [ Polypetalae 
( Metachlamydeae Sympetalae (Garaopetalae) 
The distinction between Archichlamydeae and Sym- 
petalae is not absolute, since each group contains plants 
having some features characteristic of the other. The 
Apetalae, as the name suggests, are without corolla (in 
some cases without either calyx or corolla) ; the Poly- 
petalae have sepals and petals (one or both) entirely 
distinct; while the Sympetalae have the sepals and the 
petals wholly or partly united so as to form a tubular calyx 
or corolla. In the light of our preceding study of the 
fruiting branch or "flower" of Gymnosperms, it will be 
readily understood that flowers of simple structure are 
presumably more primitive than those of more complex 
structure. The simplest flower we can imagine is an 
apetalous staminate flower of one stamen ; or an apetalous 
pistillate flower of one simple pistil. Polype talous flowers 
are more highly organized than apetalous, and may 
therefore be less primitive; sympetalous flowers are 
more complex or more highly organized and are therefore 
less primitive than either Apetalae or Polypetalae. 
The following examples will serve to illustrate 1 5 of the 
more common or more familiar families of Dicotyledons, 
out of a total of over 250. 
