HUMAN MACHINE AND THE AUTOMOBILE 357 



bacteria are microscopic plants 



pond scutn 

 is slippery 

 string's of 

 cells 



plartt 

 foimd on the 

 Kortb side of 

 trees and rocks 



amoeka. 



15 OT3<3 of 



the simplest 

 animals 



the 

 slipper- 

 animal 



^Y\ small cmiYmxls 

 V\ cause waiouria 



Some Cells Can Live 



Alone. There are 



many cells which can 



live by themselves. We 



have heard of the bac- 

 teria they are such 



forms of life. Pond 



water swarms with 



many different kinds 



of single-celled animals 



and plants, while in 



some parts of the ocean 



they are so numerous 



that they form the food 



of other larger animals. 

 Cells Form Tissues. 



But although cells all 



have a similar struc- 

 ture, they differ greatly 



in size, shape, and use 



in the human body. 



We have blood cells, 



muscle cells, bone cells, nerve cells, and many other va- 

 rieties. These cells, when they are all alike and all doing 



a certain kind of 

 work, are called 

 tissues. 



Tissues Form 

 Organs. We 

 also find in the 

 body that 

 groups of tissues 

 may have some 

 work to do to- 

 ceiis from tissues. gether. Take, 



Some cells that live alone. The plant at the 

 left is a string of cells and is magnified much 

 less than the rest of the plate. All of the 

 plants and animals shown live in the water. 



cell 



chlbroplosb 



nucleus 



cytoplasm 



! im 



one celled, animals . amoeba 



