76 CHEMICAL ENVIRONMENT 



plicated ''life cycles" comparable to what occurs in higher 

 organisms. Undoubtedly actively groioing cells are different 

 in appearance from dying and dead cells. To the interpreta- 

 tion of these differences as illustrations of distinctive "life 

 cycles" the author is sufficiently conservative to apply the 

 Scotch verdict of "not proven." The student especially 

 interested in this phase of bacteriological study is referred to 

 the work of Henrici mentioned on page 75 where he will 

 find not only an excellent discussion but also an extended 

 bibliography of the subject. 



Why Cells Divide.— Why a single cell should divide, why it 

 does not continually increase in size indefinitely is usually a 

 puzzle to students even to those who are sufficiently advanced 

 to be studying bacteriology. Without entering into a 

 philosophical discussion of all the reasons which might be 

 offered, one fundamental physical explanation only will be 

 pointed out. This applies to cells that get all their food 

 material by diffusion through a cell wall and get rid of their 

 waste in the same way. 



The building up of new cell substance, growth of the cell, 

 increase in volume, is determined by the amount of food 

 material diff'using through the surface. Volume of solids 

 increases as the cube of any dimension and surface increases 

 as the square of the same dimension. Hence volume in- 

 creases relatively faster than surface and a time in growth 

 will be reached when surface is not sufficient to provide more 

 volume. Division results in increased surface and hence 

 provides for further growth. Division is a growth mechan- 

 ism. Illustrating the above for a coccus (sphere) to make the 

 calculation simpler the mathematics is as follows : 



The actual surface of a sphere is equivalent to four great 

 circles, i. e., is 47rR^ If the sphere divides into two hemi- 

 spheres each will have one great circle newly exposed. Two 

 new great circles equal 27rR^ Hence 2irK^ is added to 47rR^ 

 for surface for the same volume. Fift}' per cent more surface 

 is available for diff'usion of food material into the same volume 

 and each hemisphere can increase to at least the size of the 

 original sphere. 



