414 



JOURNAL OF THE KOYAT, TTOH rU'l l/I'liRAL SOCIETY. 



ordinary plant breeding. The Orchid field is so wide, llie conditions 

 so difficult, and the complications so numerous, that only a practical 

 expert can carry out the necessary experiments, and even he must 

 have considerable resources of capital and experience at his command. 



In order to apply Genetics to Orchid breeding it will be necessary 

 for the Orchid breeder to make himself familiar with the first prin- 

 ciples. These principles are now universally accepted by all serious 

 students of heredity who have experimented for themselves, and they 

 can be applied equally to all kinds of plants and animals, including 

 Man himself. From the practical point of view it will not be 

 necessary for the Orchid breeder, at the outset at all events, to venture 

 much beyond these first principles. His own experiments, if based on 

 these principles and carefully carried out, will soon yield a harvest 

 of valuable data that will lead him on to more advanced work and to 

 a deeper insight into the possibilities and certainties of Orchid breeding. 



First Principles of Genetics. 



Each plant or animal is composed of many characters, most of 

 which are heritable. 



Each heritable character is represented in the germ-cells by one or 

 more factors. 



In the fertilized cell (zygote) each factor is present either in a 

 double state or a single state. 



When a double dose of the factor is present, the plant or animal is 

 called homozygous, or pure in regard to that factor. 



When a single dose of the factor is present, the plant or animal is 

 called heterozygous, or impure in regard to that factor. 



When the factor is altogether absent, the plant or animal may be 

 called zerozygous, or wanting in regard to that factor. 



Previous to fertilization the germ-cells divide, and segregation takes 

 place in regard to each factor. 



Each cell with a double dose of the factor becomes two fertilizing- 

 cells {gametes), each with a single dose of the factor. ' 



Each cell with a single dose of the factor becomes two fertilizing-! 

 cells [gametes), one of which has a single dose of the factor and the 

 other has none. \ 



Fertilization consists in the union of two fertilizing-cells {gameies),\ 

 a pollen or sperm cell from the male uniting with an egg-cell from the 

 female. 



If both paternal and maternal gametes have a single dose of the 

 factor, a zygote will be formed with a double dose of the factor, and! 

 the plant or animal will be homozygous, or pure in regard to that 

 factor. 



If, however, one of the gametes has a single dose of the factor and 

 the other has none, a zygote will be formed with a single dose of the 

 factor, and the plant or animal will be heterozygous, or impure in regar(^ 

 to that factor. 



If neither of the gametes has the factor at all, it will of course be 



