CONTENTS xiii 



PAGE 



The forms of species hybrids 227 



The vigor of species hybrids 230 



Sterility in species hybrids 234 



Partially sterile hybrids of wheat and rye 236 



Partially sterile hybrids in Nicotiana 238 



Species hybridization in CEnothera 244 



Conclusions 248 



CHAPTER XIII 

 PURE LINES 



Introductory 250 



Discovery of pure lines 250 



Conditions necessary for the existence of pure lines 255 



Isolation of pure lines from mixed populations 256 



The effect of selection within pure lines 257 



Significance of the pure line principle in breeding 259 



CHAPTER XIV 



MUTATIONS 



Introductory. 263 



Two classes of mutations 263 



Chromosome aberrations 263 



Factor mutations 263 



The nature and causes o" factor mutations 266 



Factor mutations, both germinal and somatic 268 



Vegetative mutations versus somatic segregation 272 



" Mutations " in the evening primroses 276 



PART H 

 Plant Breeding 



CHAPTER XV 

 INTRODUCTION, HISTORICAL 



Introductory 287 



The beginnings of plant breeding 287 



Pioneers in plant breeding 288 



More recent progress in plant breeding; classification of methods 291 



Mass selection 291 



Line selection 293 



Hybridization 294 



Clonal selection 298 



Organization of plant breeding work 299 



Seed and plant introduction 299 



Collections of plant breeding material 299 



Research on plant groups 300 



