26 PLANT BREEDING 



Mendel formulated a law governing the rapidity with which plants 

 tend to become fixed in their characters after the original crossing. 

 If we consider one pair of characteristics, for example, wrinkled- 

 ness and smoothness of garden peas, we find that in the first gen- 

 eration the seeds are all smooth, but in the next one fourth of the 

 individuals will be pure wrinkled peas; one other fourth will be pure 

 smooth peas; the other two fourths will bear the dominant charac- 

 teristics of smoothness, but these two fourths will really be mixed. 



In the next generation of those which are mixed one fourth 

 will come as pure wrinkled, one fourth as pure smooth, and two 

 fourths as mixed but having the dominant character of smoothness. 

 The student will see that it will take many generations to entirely 

 eliminate the mixed elements if this law continues to hold good for 

 many generations. 



Results of Mendel's Law. — It is readily seen that by the law 

 there will always be an element which is mixed and which will 

 never come true to the original type nor true to either of the pair 

 of characteristics under consideration. If yellow and pink toma- 

 toes are crossed, there will always be a number of individuals 

 showing one or the other of the parent characteristics. We would 

 expect never to be able to establish pure seed after crossing. 



It is because of this law in nature that fruit growers abandon 

 the propagation of apples, pears, peaches, plums, and other orchard 

 fruits by means of seed. Other methods of propagation are de- 

 scribed in the next chapter. When plants are propagated by buds, 

 they come more nearly true to the original form and variation is 

 eliminated. 



Limits of Mendel's Law. — Plant breeders at the experiment 

 stations in the different states and elsewhere have conducted trials 

 many of which vindicate the truth of Mendel's law. 



The usefulness of the law, however, is limited because of the 

 fact that it recognizes only one pair of characters at a time. In corn 

 breeding, for example, one dominant character and the recessive 

 character corresponding to it are all that can well be considered 

 at a time in the numerical formulas of this law. Yellow corn 

 is dominant over white corn. While we are trying to select a 

 pure type after crossing corn of two different colors, we are cer- 

 tain to lose sight of some of the other characters which are 

 perhaps more important. 



Many pairs of Characters to be Considered. — If we continue 

 the illustration of breeding, we will see that there are numbers of 



