PRINCIPAL GROUPS OF PLANTS. 133 



It should be understood that the use of hybrids in such experi- 

 mental work is simply a device to secure easy recognition of the 

 contributions of each parent to the progeny. For example, if red 

 and yellow races of corn are crossed, it is very simple to recognize 

 the color contribution of each parent to the hybrid progeny, w r hen 

 it would be impossible to separate the contribution of two yellow 

 parents. The inference is, that what is true of hybrids is true of 

 forms produced in the ordinary way, so that laws of heredity 

 obtained from a study of hybrids may be regarded as laws of 

 heredity in general. 



In the working out of Mendel's law it has been observed that, 

 while one- fourth of the progeny are like one parent, the remaining 

 three-fourths will all show the characteristics of the other parent, 

 although only one of the remaining three-fourths will breed true. 

 That is to say that the hybrids, which make up half of the progeny, 

 look like one of the parents, but all do not breed true to that 

 parent. 



In this case the character of the true pure-strain parent 

 which marks the hybrids is said to be a dominant character, while 

 the character of the other pure-strain parent is said to be a recessive 

 character, because in the hybrids its presence can not be observed 

 and can be discovered only by breeding the hybrids. 



It is only by experiment and breeding that dominant and 

 recessive characters can be determined. For instance, in the 

 culture of peas the character of being tall has been found to be 

 dominant over the character of being dwarf. This means that all 

 the hybrids will be tall, although one-fourth of their progeny will 

 be dwarf. 



Again in the pea, the character of having a round seed is 

 found to be dominant over that of having a wrinkled seed. In 

 wheat the character of being beardless is dominant over that of 

 being bearded, and again the character of being susceptible to 

 rust is dominant over that of being immune to rust. 



The infinite number of characters which complicates the study 

 of hybrids and the fact that in breeding it is sometimes the dom- 

 inant and sometimes the recessive character which is the desirable 

 one to maintain suggest at a glance the breadth and difficulty of 

 the problem. 



