320 



BOTANY 



PART I 



generations, taking account of all the individuals that result and 

 breeding from them. 



1. SEGREGATION OF CHARACTERS. This is the most generally 

 applicable of the laws or rules discovered by MENDEL and will be best 

 illustrated by an example. If a red-flowered Mirabilis jalapa be 

 crossed with a white-flowered individual one obtains a generation of 

 hybrids with uniformly rose-coloured flowers If these are fertilised 

 from one another a second generation is obtained, bat the individuals 

 of this are not uniformly coloured ; in addition to rose-coloured plants 

 pure red-flowered and white-flowered plants occur in the proportion 

 per cent of 50 : 25 : 25, i.e. in the ratio 2:1:1 (Fig. 268). When 



FIG. 2(i8. Mirahilis jalapa, alba and rosea. With the hybrid between thpm in the first 

 and second generations. (Diagram. After COBRENS.) 



fertilised from one another the pure red-flowered plants produce a red- 

 flowered progeny and the white-flowered plants also breed true ; they 

 have returned to the pure parent forms. The 50 per cent of rose- 

 coloured plants again segregates in the next generation, and .like the 

 former generation yields 25 per cent pure red, 25 per cent pure white, 

 and 50 per cent rose-coloured plants. The proportion of hybrid 

 plants thus continually becomes lessened by the return to the red and 

 white types; in the eighth generation only 0*75 per cent of hybrids 

 remain, and this small remainder continues to segregate further on 

 breeding. These results are theoretically explained since MENDEL'S 

 investigations by assuming that the sexual cells of the rose-flowered 

 hybrids are not themselves of hybrid nature, but are already segre- 



