612 IMPROVEMENT OF LIVING THINGS BY MAN 



due to the fact that though the volume grows in proportion to the 

 cube of its diameter, the surface grows according to the square of 

 its diameter. Two bricks cemented together have less surface 

 than two separate bricks. Therefore in order to live, the cell must 



get more surface. It 



cut here 



does this by cell divi- 

 sion. In our study of 

 the amoeba, we saw 

 that a single cell formed 

 two cells by division. 



Animals and plants 

 grow larger by a multi- 

 plication of the cells. 

 The growth of roots, 

 buds, leaves, and 

 seeds, the develop- 

 ment of a kitten, and 

 the shooting up of a 

 boy or a girl at ado- 

 lescence are examples 

 of rapid cell division. 

 New plants may be 

 formed by cell division. 

 It is a matter of com- 

 mon knowledge that 

 when a chestnut or a 

 poplar tree is cut 

 down, young shoots 

 come up from around 

 the cut stem. New 

 geranium plants may 

 be grown from " slips" or cuttings from the old plant. Sugar 

 cane and banana plants are reproduced entirely in this way, not 

 from seeds. Grape vines, strawberry plants, and many others 

 take root where the stem comes in contact with the soil. Many 

 grasses, like the couch or quick grass, become pests because they 

 spread with such rapidity from their underground stems. Ferns 



Regeneration of a flatworm. The shaded area shows the 

 newly regenerated parts. 



