14 TEXTBOOK OF BOTANY 



22. Growth. In the hay infusion we find bacillus cells of 

 different lengths. This is because a cell does not remain 

 the same size, but if it is supplied with food, and if other 

 conditions, such as moisture and temperature, are favorable, 

 it grows. Growth goes on because the cell builds up into 

 living matter some of the food that it takes in, and this be- 

 comes a part of the cell body, just as the food that we eat is in 

 part built up into the substance of our own bodies. It is true, 

 as we have just seen, that the opposite process also goes on ; 

 that is, the living matter is at the same time being broken 

 down into simpler substances by respiration, and these sim- 

 pler substances are given off to the outside through the cell wall 

 as waste matter. If the building up and tearing down should 

 just balance each other, the cell might remain of the same size. 

 But if food is abundant, as it is in a hay infusion, the 

 building up goes on more rapidly than the tearing down, and 

 the cell grows. 



23. Reproduction. The growth of a cell does not go on 

 indefinitely. When it reaches a certain size (which is always 

 about the same for any particular species so long as the condi- 

 tions remain unchanged) it can grow no longer, but divides. 

 The division of the cell is always crosswise, so that the two 

 smaller cells thus formed are attached end to end. These 

 may remain attached, and each one then grows to full size 

 and divides, and by this alternate growth and division rows 

 or colonies of cells are formed. The cells, however, are easily 

 broken apart, so that in an infusion we always find single 

 cells as well as colonies of various lengths. It is by division 

 that the bacillus reproduces, and cell division is the only kind 

 of reproduction that is found among bacteria. Many bac- 

 teria are able to grow and divide very rapidly ; some kinds 

 have been observed to divide about every half-hour. In 

 such a case a single cell might give rise in the course of ten 

 hours to 1,048,576 cells; and in the course of twenty-four 

 hours, if the food supply were abundant and all other con- 



