324 Plant Life and Evolution 



HEREDITY 



Chromosomes the Bearers of Heredity. The 

 mystery of heredity has always aroused the interest 

 of biologists, and many ingenious theories have 

 been propounded to explain it, but all of these are 

 more or less unsatisfactory, as they assume premises 

 which are impossible of demonstration. The facts 

 of fertilization, as they have been studied in both 

 plants and animals, indicate that the sexual nuclei 

 are undoubtedly the most important parts of the cell 

 in the sexual process, and Strasburger believes that 

 in the higher plants the chromosomes alone are po- 

 tent as the bearers of heredity, since only the sexual 

 generative nucleus from the pollen-tube enters the 

 egg-cell. We must remember, however, that in the 

 lower plants the whole protoplasts of the gametes 

 fuse, cytoplasm as well as nuclei. Moreover, it must 

 not be forgotten that in very many plants inheri- 

 tance is through nonsexual methods, principally by 

 budding, where there is no development of special 

 reproductive cells as is the case in sexual repro- 

 duction. 



If the chromosomes are allowed to be the vehicles 

 of transmission of hereditary characters, it still 

 remains to be explained how this is accomplished. 

 The assumption that they contain innumerable 

 " determinants " which correspond to the special 

 structures of the adult, is a simple explanation, but 

 not a satisfactory one. De Vries' theory of " pan- 



