202 INTRODUCTION TO BOTANY 3 
is soon exhausted, and the excretions from the bacteria them- 
selves interfere with their growth and reproduction. But in 
situations where bacteria can grow they really do reproduce 
themselves and increase their number with astonishing rapidity 
(fig. 166). The possibilities of production and growth of large 
Fig. 166. Gelatin culture of bacteria 
The sterilized gelatin in the dish was exposed in the hallway of a schoolroom for 
five minutes, and then kept covered for five days, during which time the different 
colonies of bacteria grew. Each colony grew from one, or possibly, in some cases, 
from a few individual bacteria, and the number of bacteria that have grown in 
each colony is so large that the colony is easily seen 
numbers are evident when we remember that many millions 
of bacteria could live for a considerable time in a cubic inch 
of milk or beef broth. 
In a few kinds of bacteria the interior of the cell sometimes 
becomes rounded and inclosed by a relatively heavy wall. 
This heavy-walled body may remain inactive for a long period, 
