138 PRINCIPLES OF HEREDITY 



when it loses its power for further growth and division. The 

 reduction division and the subsequent formation of the gametes 

 results in the removal of substances that prevent their continued 

 activity and as a consequence they are restored to a youthful 

 condition which appears in the growth of .the gametospore. 



Much attention has been directed in recent years to the process 

 of fertilization as a method of controlling the character of off- 

 spring. You have already noted, page 56, that granular bodies 

 or chromosomes are constant features of every nucleus. These 

 complex bodies determine the type of plant that shall be de- 

 veloped. In other words the chromosomes contain the hereditary 

 substances, termed factors, that cause each plant to develop in a 

 certain definite way or to follow a certain pattern in its growth. 

 The chromosomes derived from the parent plants cause the off- 

 spring to resemble them in growth. The micro- and mega-spores 

 are formed in the sporophylls, and these spores must therefore 

 contain the same kind of chromosomes or factors as the plants 

 bearing the sporophylls, i. e., the parent plant, since they have 

 been derived directly from the parent plant by cell division. 

 Therefore gametes derived from the germinating spores must 

 also contain the same factors as the parent plants. In fertiliza- 

 tion we have the fusion of the gametes. This means that the 

 hereditary substances in the gametospore will be of a dual 

 character since it has been derived from the male and female 

 gametes (Fig. 80). Consequently there is now lodged in one 

 body, the gametospore, all the possibilities of growth that each 

 gamete inherited from the parent plant. 



Let us now examine some of the evidence indicating that the 

 hereditary substances or factors are distributed in a very definite 

 way from parent to offspring and that these factors are associ- 

 ated with the chromosomes. In the common garden plant, four- 

 o'clock, we find some plants bearing only white flowerS; others 

 only red flowers. Here we have evidence of two factors; the one 

 causing the white color, the other causing the red color. If these 

 two plants be crossed the resulting offspring is pink or inter- 

 mediate between the two parents. This offspring is a hybrid, 

 meaning thereby an individual derived from parents that differ 



