CHAPTER XV 



PHYLOGENY OF ANGIOSPERMS 



The phylogeny of any great group will probably always 

 remain a baffling problem. At the same time, theories of phy- 

 logenv serve to coordinate knowledge and stimulate investiga- 

 tion. The phylogeny of Angiosperms is an unusually obscure 

 problem. The hypotheses proposed seem to include almost 

 every possibility, but thus far they have been more interesting 

 than convincing. When similarity of structure was taken as a 

 sure indication of genetic relationships, the problem promised 

 an approximate solution. But since it has been proved that 

 similar structures may develop independently, the difficulty of 

 solution has apparently become insurmountable. Under such 

 circumstances it is questionable whether a discussion of the sub- 

 ject is profitable, but a statement of the problem may not be 

 out of place. 



The first phase of the problem has to do with the common 

 or independent origin of the Monocotyledons and Dicotyledons. 

 It has been assumed generally that the two groups are mono- 

 phyletic. The chief argument, and in fact the only morpholog- 

 ical one for the monophyletic theory, lies in the great uni- 

 formity of the peculiar development of both the male and 

 female gametophytes. It is argued that the independent 

 origin of such exact details of development and structure 

 is inconceivable, and this argument has been reenforced re- 

 cently by the discovery in both groups of the peculiar phe- 

 nomenon called " double fertilization." The argument is cer- 

 tainly a very strong one. and yet there are rebutting proposi- 

 tions. Even such similarity in structure may be the natural 

 outcome of the changes that resulted in the evolution of seeds, 

 and these are now generally believed to have appeared in inde- 

 280 



