ii8 Plant Genetics 



complex material. Results have been given in experi- 

 mental work with peas, four-o'clocks, wheat, and, most 

 of all, corn, all of them seed plants. This selection of 

 material was natural, for such plants represent the 

 region of the plant kingdom in which practical breeding 

 occurs. It was natural to make practical breeding 

 scientific. 



Not only have geneticists used the most complex 

 material but they have selected sexual reproduction as 

 the method of reproduction to be investigated, which is 

 the most complicated kind of reproduction. This selec- 

 tion was also natural, for Mendel's law is based upon 

 sexual reproduction, and it is this law that geneticists 

 have been testing. 



The further difficulties of the situation should also 

 be considered in order that one may be in a position to 

 estimate the value of results. Students of morphology 

 are familiar with the fact that the sexual structures of 

 seed plants are peculiarly involved with other structures. 

 They do not stand out distinctly, as in the algae, for 

 example. Not only are the sexual structures (eggs and 

 sperms) beyond the reach of observation, and therefore 

 of experimental control, but there is the alternation of 

 generations to consider, inheritance being carried through 

 one generation to express itself in the next. A sporo- 

 phyte does not beget a sporophyte but a gametophyte, 

 and this in turn begets the embryo sporophyte. 



Add to this complex situation the possibility of 

 parthenogenesis and vegetative apogamy and it is ob- 

 vious that the origin of embryos found in seeds is not 

 assured. If two embryos appear it is evident that at 

 least one of them holds no relation to the pollen parent ; 



