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 
