CONTENTS 
The forms of species hybrids 
The vigor of species hybrids . 
Sterility in species hybrids : 
Partially sterile hybrids of wheat and 3 rye . 
Partially sterile hybrids in Nicotiana . 
Species hybridization in GEnothera . 
Conclusions . 
CHAPTER XIII 
Purse Lines 
Introductory . : 
Discovery of pure lines . 
Conditions necessary for the existence oe pure ines : 
Isolation of pure lines from mixed populations . 
The effect of selection within pure lines . 
Significance of the pure line principle in ieodings 
CHAPTER XIV 
MutaTIons 
Introductory. . 
Two classes of enter Wine 
Chromosome aberrations 
Factor mutations. 
The nature and causes of factor cae 
Factor mutations, both germinal and somatic . 
Vegetative mutations versus somatic segregation . 
“Mutations” in the evening primroses 
PART II 
Plant Breeding 
CHAPTER XV 
IntrropucTIon, HisToRICcAL 
Introductory . 
The beginnings of ne preedines 
Pioneers in plant breeding . 
More recent progress in plant breeding; eaaeeion of Perhode : 
Mass selection . 
Line selection 
Hybridization . 
Clonal selection 
Organization of plant breeding sets 
Seed and plant introduction . 
Collections of plant breeding meee 
Research on plant groups . 
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