254 Practical Plant Biology. 



resulting plant. But the complicated procedure not only secures 

 that the bulk of nuclear material received by each cell shall be 

 equal, but by the fission of the individual chromomeres it provides 

 that the nuclei of every cell of the mature plant will contain a 

 representative of every chromomere of the nucleus of the oosperm. 

 Hence, even if the chromomeres differ among one another, mitosis 

 secures that the resulting nuclei shall be equivalent to one another, 

 not only quantitatively but also qualitatively. 



The elaborate mechanism adopted for the accurate division of 

 the nuclear substance naturally arouses speculation and forces us 

 to consider what the nature of this substance must be which calls 

 for such meticulous distribution throughout the cells of the 

 organism. The cleavage and distribution of the chromosomes or 

 rather of the chromomeres seem to be the essential result aimed at 

 in mitosis, and any satisfactory hypothesis must take this fact into 

 account. The generally received view is that the chromatin is the 

 hereditary substance, or germ-plasm, of the organism. It controls 

 and directs the development of the cells and secures that they shall 

 have the characteristics of the species to which the individual 

 belongs, and shall contain representatives of the distinctive traits, and 

 features which characterised that individual's parents. The separ- 

 ate characteristics and traits, or groups of them, are probably re- 

 presented by portions of this substance, viz. the chromosomes and 

 chromomeres. Thus the chromatin of the oosperm being distri- 

 buted throughout the cells of the embryo compels the development 

 of the latter into an organism like its parents. This hypothesis 

 fits in well with the observation that, given the necessary condi- 

 tions, each and every cell of a plant or animal appears able to 

 give rise to a complete organism, containing as it does in its 

 nucleus the complete germ-plasm. The propagation of plants by 

 buds, cuttings and offsets furnishes familiar examples of this phe- 

 nomenon. 



When the oosperm is formed by the fusion of the sperm and 

 ovum, it receives the nuclei of two individuals, and consequently 

 the individual produced must, if our theory be correct, combine 

 the characters of the two. Thi's surmise is justified by observation. 

 Furthermore, the fact that the offspring resembles the male parent 

 as much as the female, constitutes one of the most cogent argu- 

 ments that the substance of the nucleus is the germ-plasm. For 

 the sperm contributes little else than chromatin, and the bulk of 

 its chromatin is approximately if not exactly equal to the bulk of 

 the chromatin contained by the ovum. 



The hypothesis further necessitates that the oosperm must con- 



