VEGETATIVE REPRODUCTION 205 



selves more or less intimately connected with 

 an abundant supply of nutrition. 



The rapidity with which many of these 

 simple plants can multiply, provided the 

 nutrition conditions are favourable, is truly 

 astonishing. Instances are not uncommon, 

 especially among bacteria, in which a cell 

 colony will double its numbers every twenty 

 minutes or so. That is to say, in about 

 twelve hours one cell might give rise to nearly 

 seventy thousand million cells. It is highly 

 improbable that anything approaching this 

 number would actually be reached, because 

 as the colony begins to grow the individuals 

 composing it compete with each other for the 

 food supply, and those more centrally situated 

 will obviously be at a disadvantage in this 

 respect. Many other conditions, also de- 

 pending on the crowding of the cells, will 

 begin to make their effects felt on the repro- 

 ductive capacity of the members of different 

 portions of the colony. 



Now what is true of a colony of detached 

 individuals is still more applicable as soon as 

 the dividing cells cease to separate at once 

 from each other. This naturally follows from 

 the simple geometrical fact that if the cells 

 are all growing and dividing equally and in 

 all directions, the surface of the cell colony 

 only increases as the square of the radius of 

 the growing spherical mass, whilst its mass 

 increases as the cube. The difference in 

 available nutrition evidently must affect the 



