304 ESSENTIALS OF BOTANY 
the lower ones, he would probably put some one of the 
green alge at the bottom and the angiosperms at the top 
of the series. 
385. The Oldest Angiosperms. — It is impossible to give 
any of the reasons for the statements of this section with- 
out making an unduly long chapter. It is not yet certain 
whether monocotyledons or dicotyledons were the first to 
appear on the earth. The descent and various relation- 
ships of the families of dicotyledons can be discovered by 
the study of the flower, fruit, and seed better than by the 
examination of the vegetative organs. 
The entire pedigree of the several families cannot be 
represented by arranging the names of the families in a 
straight line. Their relationships can be shown, however, 
in a general way, and a part of the pedigree is indicated 
by the succession of families in the Flora which accom- 
panies this book. The Willow family is perhaps the oldest 
of the more familiar families of dicotyledons, and the Com- 
posite family the youngest. The beginnér must remember 
that it is not such “typical” flowers as are shown in 
Figs. 97 and 98, but rather bilaterally symmetrical ones 
like Fig. 101, or epigynous and closely grouped flowers like 
those of the Composite family (Appendix I, Fig. 8), that 
represent the highest development among flowers. 
386. Division of Labor and Plant Evolution. — A little 
has been said in Sect. 384 about the steps of development 
in the reproductive processes of plants. In general the 
more complicated kinds of reproduction are found among 
the younger and higher types of plants. The same gen- 
eral law applies to the vegetative parts of the plant body. 
The lowest undoubted plants (Figs. 150, 169) show very 
little division of labor among their parts. A single cell 
