HEREDITY AND VARIATION 225 



accordiiis to (lofinit(> laws. ThcM-o is still a third type of variation, 

 often of importan(!e in nature and in j)ractical breeding, which 

 we know as mutation. Frequently an individual clearly different 

 from the rest will appear in a pure race, where there is apparently 

 nothing in the ancestry which can explain its origin, and will 

 breed true to this new type in succeeding generations (Fig. 127). 

 The production of such a new and distinct type we call a muta- 

 tion. Many "double-flowered " races of plants have arisen in this 

 way, as have forms with cut leaves and a great variety of 

 other characters. When the history of such a plant type can be 

 traced, it is often found to begin with a single individual which 

 arose by mutation from the normal race and has transmitted 

 its characters to its descendants. In some cases a mutating 

 individual is strikingly different from the normal form and is 

 then often called a "sport". In others, the difference is so 

 small that it can hardly be recognized. Many instances have 

 also been found where the mutation appears in a single branch 

 or portion of the plant rather than in a whole individual grow- 

 ing from seed (Fig. 128). All mutations agree, however, in 

 coming without warning or evident cause and in being trans- 

 mitted to offspring. By mutation have arisen some of our 

 important horticultural and crop plants (Fig. 129) such as the 

 kohl-rabi, the navel orange, the thornless cactus, the moss rose, 

 the Shirley poppy, and others. We can understand and man- 

 ipulate, to a certain extent, the variations due to environment 

 and to hybridization, but mutations are as yet beyond our con- 

 trol. The best that the plant breeder can do is to watch for 

 them closely and seize upon them when they appear. 



The science of genetics is today one of the most intensively 

 studied branches of biology and has not only yielded us valual^le 

 information as to the laws by which various characteristics are 

 transmitted from parent to offspring, but through its identifica- 

 tion of the chromosomes of the nucleus as the probable seat of 

 genetic factors, it has even thrown light on the structure and 

 behaviour of protoplasm itself. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



577. What is the chief practical importance of discovering laws of 

 inheritance? 



578. In studies of inheritance in the summer squash it has been 

 found that white fruit color is dominant over yellow, and that the differ- 



15 



