456 STRUCTURE AND LIFE HISTORIES 



in dicotyledons the parts are typically in fours or fives. 

 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: 



f Apetalae 



_. ^ , , Archichlamydeae < rt , >. i 



Dicotyledons < [ Polypetalae 



[ Metachlamydeae Sympetalae (Gamopetalae) 



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. Polypetalous 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. 



