INHERITANCE 715 



ferent periods of the life history there are very great differences in form, 

 structure, and physiology, constituting an "alternation of generations." 

 Each condition is transmitted from parent to offspring for many genera- 

 tions, yet each in time transforms into a later condition. 



All these phenomena are commonly thought of as matters of "life 

 history," rather than of inheritance. Yet they represent fundamental fea- 

 tures in those relations of successive generations that are called inherit- 

 ance. The single cell, reproducing vegetatively, produces a great number 

 of other free cells that are like itself in their special peculiarities. Later 

 the character of the cells changes; and again the resulting condition is 

 for a long period inherited in vegetative reproduction. In these respects 

 the phenomena are like modifications resulting from environmental 

 action, as shown in later paragraphs. 



Are the diverse conditions that are vegetatively inherited in different 

 periods — such as sexual immaturity and maturity — the result of changes 

 in chromosomal materials, or changes in the cytoplasm? Dobell (1924) 

 shows that the chromosomes do not visibly change in the series of 

 diverse forms passed through in the life history of certain haploid 

 Sporo2oa; throughout all the changes the same set of chromosomes in 

 the same number are present. Tartar and Chen (1940) have found that 

 in the period of sexual maturity, in P. bursaria, parts of the individual 

 consisting only of cytoplasm react sexually. Neither of these observations 

 proves conclusively that the chromosomal materials are not altered in the 

 different periods, but they perhaps make it probable that the different 

 periods in the life history result rather from such interactions between 

 chromosomes and cytoplasm as must occur in producing the bodily differ- 

 entiations of a developing multicellular organism. 



Whatever the seat of the different inherited conditions in different 

 periods of the life history, it is clear that the material on which the 

 different conditions depend must multiply itself, for long periods remain- 

 ing true to type. An immature individual contains a certain small amount 

 of the material on which immaturity depends. In ten generations this 

 material has multiplied to more than a thousand times its original quan- 

 tity, still remaining immature. Later, having attained the mature condi- 

 tion, it again multiplies in that condition to thousands of times its 

 original quantity. X^V^'^'I/ 



^ 



