32 LABORATORY LESSONS IN GENERAL SCIENCE 



plants and animals is the material that is alive. Just what 

 this "life" may be, making the difference between living 

 and dead tissues, remains unknown to scientists. 



2. Growth of the tissues is commonly accomplished by 

 the subdivision of living cells, apparently from causes 

 (activities) wholly within the cell itself. These new cells 

 are like the original cell, and are in turn capable of sub- 

 division. In the case of the microscopic plants known as 



FIG. 16. Successive stages in the subdivision of a cell. 



bacteria, subdivision under favorable conditions is said to 

 occur as frequently as every twenty or thirty minutes. 

 Calculate in round numbers how many of these one-celled 

 bacteria could result in twenty-four hours from a single cell 

 where subdivision of every cell occurred every half hour. 



3. In the higher orders of plants, and in animals, the for- 

 mation of a more complex cell structure is far slower than 

 this, and in active animals like man a fairly even balance is 

 maintained throughout much of one's life between increase 

 in new cells and the destruction of others. Why may any 

 considerable loss in weight, during middle life, or failure 

 during youth to increase somewhat regularly in weight,' 

 be a cause for concern ? 



4. Activity in some form is an accompaniment as well as 

 an evidence of life. A low degree of physical or mental 

 activity is oftentimes evidence of a lack of sound conditions 



