FUNCTION OF THE NUCLEUS 67 



not, the part containing the nucleus regenerates, i.e. 

 it grows again into a complete cell. The parts not 

 containing the nucleus remain for a time sensitive and 

 motile, but they cannot assimilate food, and hence 

 they ultimately die. In living plant cells, when the 

 protoplasm is in a state of great activity and metabolism 

 is proceeding rapidly, the nucleus is large and con- 

 spicuous (Fig. 43, A, B, C), but when the cell is com- 

 paratively inactive the nucleus is usually small and 

 inconspicuous. Again, when local activity is going on 

 in a cell the nucleus commonly moves to the spot in 

 which work is being carried out. For instance, if a 

 plant cell is thickening its cell wall in one spot the 

 nucleus moves to the spot where the thickening is 

 going on and remains there while this activity is in 

 progress (Fig. 43, E). It appears to be " superintending 

 operations." 



From facts like these we infer that the nucleus is 

 essential in directing the metabolic and formative cell 

 processes. Exactly what it does we do not know, but 

 very likely it sends out into the cytoplasm chemical 

 substances, perhaps enzymes, which are essential in 

 carrying out the work of the protoplasm. 



The chromatin of the nucleus is certainly different 

 in every species of organism, and even in those smaller 

 groups of very uniform individuals within the species 

 which are called " pure strains." The differences 

 probably depend on the existence in the chromatin of 

 slightly different proteins (cf. p. 43). The cytoplasm 

 of different species and strains also probably differs in 

 the same sort of way, but we have very good evidence 

 that the differences of the chromatin are the most 

 important. The peculiar formative powers of each 

 species, the powers that enable it to reproduce the 



