INHERITANCE— IMPROVE ME XT BY BREEDING 163 



When we plant seed of Hickory King maize we expect CHAP, 

 to harvest a crop of white Hickory King, and when we plant 

 seed of Yellow Horsetooth we expect to harvest Yellow Horse- 

 tooth grain ; in other words, we recognize that in plants, as in 

 animals, certain characters are inherited from the parent and 

 again transmitted to the offspring, from generation to genera- 

 tion. This is such a general rule that it is the exceptions to it 

 which cause remark. 



124. Mechanism of Transmission. — Although it is the 

 gamete which carries the power of reproducing the parental 

 characters, it is not yet known what part of the cell mechanism 

 actually takes part in the process. As it is the nuclei of the 

 two cells which unite in fertilization, they must be associated 

 with the process of transmission. It has been suggested fre- 

 quently that the chromosomes of the nucleus are the actual 

 conveyors of the separate characters, but many botanists and 

 geneticists see objections, apparently insurmountable, to the 

 " chromosome theory " as it has been called, and the subject 

 is still one of investigation. In the case of maize the small 

 size of its chromosomes makes their study difficult, and little is 

 known about them. 



125. The Zygote. — The term zygote was originally applied 

 to the single cell produced by the union of two gametes (IT 123). 

 But since this cell, by growth through somatic division (': 123), 

 develops into a complete plant bearing the characters of the 

 original zygote, the term is often extended to the plant de- 

 veloping from that zygote. 



As the zygote may be derived either from gametes which 

 are alike as regards contained characters, or from gametes 

 which are dissimilar, they may be either pure-bred or cross- 

 bred. The pure-bred zygote, formed by the union of two 

 similar gametes, is called a homosygote ; that derived from the 

 union of dissimilar gametes is called a heterosygote. 



126. The Homosygote. — The homozygote is the progeny of 

 parents similar in character, it is therefore like them as regards 

 those characters which are common to the two parents, and 

 its progeny are also like them and continue to breed true. 



It is probable that, strictly speaking, there is no such thing 

 as a regularly domesticated plant which is homozygous (i.e. 

 pure-bred) for all its characters. We usually use the term with 



