GROWTH 181 



ished. We have no reason to think that the osmotic force 

 is less, for the turgor is not lower in large than in small 

 cells. So it cannot be merely the ratio of surface to mass 

 which is the determining factor. 



The high turgor and osmotic force maintained in large 

 cells imply, besides the density of the cell-sap, that the 

 enclosing membranes of the cells are not freely permeable. 

 Upon the impermeability of the membranes and upon the 

 composition of the cell-sap depend the turgescence, the 

 plumpness, of cells . of large size. The membranes must 

 possess strength as well as impermeability, but it is their 

 impermeability which makes it necessary that they should 

 also be strong. It is this change in the quality of the sur- 

 face rather than in the mere ratio of surface and mass, 

 which is the important factor in limiting size. But this is 

 not all. 



Whatever may be the distinct functions of nucleus and 

 cytoplasm, it is safe to conclude that neither can be greatly 

 increased or diminished in amount or in activity without 

 affecting the other and the cell as a whole. By chilling cul- 

 tures of Spirogyra while in a state of cell-division, Geras- 

 simow * regularly secured the formation of cells without 

 nuclei, normal cells, and cells with double the usual amount 

 of nuclear substance. He found that the non-nucleated cells 

 grew very slightly in length, that normal cells grew nor- 

 mally, that cells with more than the normal amount of nu- 

 clear substance attained a larger size and divided later than 

 normal cells. Since the volume of the nucleus does not keep 

 pace with the volume of the cytoplasm or of the cell as they 

 both increase shortly after the cell is formed by division, 

 the disparity in the amounts of nuclear and cytoplasmic 

 substances increases. It is conceivable that growth ceases 

 when the amount of cytoplasmic in proportion to nuclear 

 substance has attained the optimum or maximum ; in other 

 words, that the limit of growth is fixed in the first and fun- 

 damental stage, the subsequent increase in size going only 

 to the limit set by the amount of protoplasm formed. 



* Gerassimow, J. J. tJber den Einflues des Kerns auf das Wachsthum 

 der Zelle. Moskau, 1901. 



