CHAPTER XI 

 HEREDITY AND VARIATION 



As a result of the process of reproduction which we have 

 described in the preceding chapter, a continuous succession of 

 new individuals arises. One of the most remarkable features of 

 this reproductive activity is that each of these new individuals 

 bears a very close resemblance to its parents. The offspring of 

 wheat plants are always wheat plants and nothing else; and the 

 offspring of oak trees are always oaks. Furthermore, any 

 particular kind or variety of wheat or of oak will produce (under 

 proper conditions) plants of that kind or variety. This tendency 

 for offspring to display the particular characteristics which 

 distinguish their parents is called heredity. 



Heredity. — We have already noted the exceedingly narrow 

 physical bridge — the reproductive cells or gametes — which con- 

 nects one generation with the next. To its offspring, one parent 

 contributes a single male cell and the other parent a single egg 

 cell, and out of the fertilized egg arising from the fusion of these 

 two gametes, the new plant develops (Fig. 118). It is evident, 

 therefore, that the parental characteristics must in some way be 

 transmitted in the protoplasm of these tiny sexual cells. Any 

 actual plant character (such as redness of flower or tallness 

 of stem) obviously cannot be found in these cells; but something 

 representing it, and capable of producing it in the new plant, 

 must be there. This "something" — we are still ignorant of its 

 real nature — is called the factor or gene for the character in 

 question. In a wheat plant, let us say, the height and strength 

 of the stem, the shape and texture of the leaf, the number of 

 spikelets in the head, the shape, color and surface of the glumes, 

 the weight of the kernel, the character of the grain, the yield of 

 seed, the resistance of the plant to cold, drought and disease, 

 together with a host of other characteristics, have all been shown 

 to be clearly inheritable. It is evident that in every male gamete 

 and in every female gamete there must be a factor which repre- 

 sents each of these characteristics and which thus determines the 

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