Spirogyra Cells 205 



trol the amount of materials, through the fact that as- 

 similation is limited by the area of chlorophyll exposed 

 in the spiral bands. 



It has been customary to regard the stiff wall of a 

 plant cell as controlling cell size ; yet it is the cytoplasm 

 that determines where the cell wall shall lie, and, as 

 growth of the cell proceeds how much the wall shall 

 enlarge. We know nothing about the question whether 

 the size of the nucleus or of the cytoplasm or of the cell 

 sap or of the cell wall comes first. All are to be ac- 

 counted for, and the account is hardly complete as long 

 as they are put merely in terms of one another. 



Equality of fission. The second fundamental study 

 of size which has been carried out satisfactorily only 

 upon Spirogyra cells is this: When a cell divides into 

 two cells, how accurately is its substance distributed to 

 the daughters ? It might have been preferable to start 

 this book with this question instead of ending with it. 

 A very few data upon the sizes of sister individuals at 

 their formation from the parent were obtained by 

 Simpson ('02), Jennings ('08b) and Popoff ('09) upon 

 Paramecium, but their number is insufficient to treat 

 statistically. 



Gerassimow ('01, page 192) compared the lengths 

 of 29 pairs of daughter cells. In no case was the total 

 difference between them more than 14 per cent of their 

 mean length; the coefficient of variation was 3.9 per 

 cent. 



A much larger series was measured by VanWisse- 

 lingh ('20). His individual frequencies are plotted in 

 figure 66. With these the liberty has been taken to 

 average each five groups. The average frequencies 

 then fit exactly a logarithmic curve which is a half of 

 a normal frequency curve. VanWisselingh fitted a 

 hyperbola to the data; this curve agreed poorly, and 

 was of a type rarely found in statistical distributions. 



