142 PLANT GROWTH 



nized by man from the period of earliest history to the 

 present. For this reason, selection of desirable plants and 

 animals for use in reproduction has been practiced since 

 prehistoric times. 



We speak of such resemblance of the parent and offspring 

 as evidence of a close fundamental relationship by saying 

 "a chip of the old block," "blood will tell," or "like begets 

 like." Actually much remains to be done by the men of 

 research before the whole story of heredity can be explained, 

 but many of the old ideas are remarkable for their accuracy 

 considering the knowledge of that day of cell structure and 

 reproduction. If chromosome structure in Chapter 4 and 

 sexual reproduction in Chapter 10 are studied, it will be clear 

 that the sex cells might be called "chips" and that the 

 chromatin is a special kind of protein that passes from cell 

 to cell during their divisions, from generation to generation. 



Heredity and variation are so closely associated in repro- 

 duction and development that it is difficult to treat them 

 separately. Heredity is the transmission of the character- 

 istics of the parents to the offspring. If this law alone oper- 

 ated, all organisms would be alike. However, it will be seen 

 from the discussion of fertilization in the previous chapter 

 that the chance of an individual's having the same set of 

 dominant genes as either parent is very remote. For the 

 same reason the offspring of the same parents are all differ- 

 ent, but all resemble the parents in some of the characters. 

 Only in identical twins and in asexual methods of reproduc- 

 tion should the genes be the same in two or more individuals, 

 and asexually produced individuals do resemble each other 

 more than do individuals that are produced sexually. The 

 several peas in a pod each come from a different pair of sex 

 cells and therefore produce plants which differ in some of 

 their characters. The small link of protoplasm (the sex 

 cells) connecting the parents with the offspring bears the 

 chromosomes which are the physical basis of heredity. 



