216 



HEREDITY AND SEX 



structive. His results and his theory can best be 

 given together. 



Correns assumes that each plant contains some factor 

 that produces a secretion on the stigma of the flowers. 

 This secretion inhibits the pollen of the same plant 

 from extending its pollen tube. He found, in fact, 

 that the pollen grains do not grow when placed on the 

 stigma of the same plant. All plants will be hybrid 



9 B ^ 



(3 



r 



%• B 



4- 



4tifG 



■f- 



-i- 



FiG. 104. — Illustrating the crossing of the types Bb and Gg to give four 

 classes : BG, Bg, bG, bg. Each of these is then back-crossed either to B or 

 to G with the positive (+) or negative (-) results indicated in the diagram. 



for these factors, hence plant B will produce two kinds 

 of germ-cells, B and h. Similarly, plant G will produce 

 two kinds of germ-cells, G-g. If these two plants are 

 crossed, four types will be produced. When these are 

 back-crossed to the parents, the expectation is shown in 

 the diagram (Fig. 104). Half the combination should 

 be sterile and half should be fertile. This is, in fact, 

 what occurs, as shown in the same diagram. The 

 - signs indicate that fertilization does not occur, while 

 the -t- signs indicate successful fertihzation. 



Correns' theory is also in accord with other com- 



