68 PRINCIPLES OF AN I. UAL BIOLOaV 



pieces, have lost the power of producing new nienibrancs; and thouj;h 

 they may continue to i)ro(hi('e starch by photosynthesis, this starch 

 cannot hitor be utihzed. 



Evidence From Position of Nucleus. — The position of the nucleus 

 in cells that are physiologically very active also strongly suggests the 

 constructive function of the nucleus. In plant cells in which the cell 

 wall on one side is being thickened, the nucleus is usually found adjoining 

 the thickening wall; and observations have shown that the nucleus 

 proceeds to that position before the thickening of the wall begins. In 

 cells which are producing root-hairs by the outgrowth of the wall, the 

 nuclei arc generally at or near the point of growth. 



Summary and Inlerence. — In each of the cases of cell fragmentation 

 mentioned above, the non-nucleated piece lacked some capacity for re- 

 construction. The missing parts of Oxytricha and Stentor could not be 

 replaced; the shell of Polystomella was not repaired; digestion ceased in 

 Amoeba, probably owing to failure to produce the necessary enzymes. 

 In plasmolj^zed cells the non-nucleated portions do not produce a cell 

 wall or membrane, nor the enzymes with which to utilize the starch 

 which their chloroplasts manufacture. In highly active cells the nuclei 

 are at the places of greatest activity. The conclusion is scarcely to be 

 avoided that, while neutral or destructive processes, like movement, 

 respiration, or excretion, may go on in the absence of a nucleus, the 

 nucleus has some very fundamental control over the constructive 

 processes of the cell. The influence of the nucleus may be exerted 

 through its supposed effect in increasing oxidation, the rate of oxidation 

 being too slow in the absence of a nucleus to permit of the syntheses 

 mentioned. 



There is abundant evidence from other sources that the portion of 

 the nucleus which exercises this control is the chromatin. The minute 

 accuracy of the behavior of the chromatin in cell division and the facts 

 of development and heredity indicate that this substance is of unusual 

 importance, but consideration of this evidence must be deferred to later 

 chapters. 



How the nucleus, or the chromatin in the nucleus, if that is the impor- 

 tant substance, exercises this control over the activities of the cell is not 

 known. Some have held that material particles of chromatin pass out 

 from the nucleus into the cytoplasm at intervals, and thus bring about 

 the regulation that the nucleus performs. In support of this view are 

 observations on some of the Protozoa, preparations of which oc- 

 casionally have the nuclear membrane ruptured and a mass of 

 deeply staining granules near the point of rupture. Such an occurrence 

 has even been reported for the many-celled animals by some observers, 

 but other competent investigators of wide experience have been unable 

 to verify the claim. It seems more likely, therefore, that the chemical 



