I 



254 LIVING THINGS IN THEIR ENVIRONMENT 



adding sugar from the outside. Living things grow 

 not from the outside, like crystals of sugar or ice, or the 

 beautiful stalactites in a cave, but from the inside. As we 

 shall see later, living things have the power to take in food 

 and change it to the living stuff out of which they are 

 made. 



Living Things Are Built out of Cells. Another charac- 

 teristic of living things is that they are made up of tiny 



J ,, units of material 



called cells. If very 

 thin slices of a plant 

 stem or bits of onion 

 skin be examined un- 

 der a microscope, 

 they will be seen to 

 be made up of tiny 

 structures such as 

 you see in the dia- 

 . gram. These cells 



The cells of onion skin seen under a compound 



microscope. The oval structures in the cells are have Various charac- 



the nuclei. How many nuclei (singular, nucleus) to teristic shapes in an- 



eachcell? i j i u 



imals and plants, but 



they are always very small. They grow by dividing, 

 forming groups of like cells, to which we give the name 

 tissues. In the stem of a plant we find several different 

 forms of cells, some with heavy walls, so that they may 

 support the stem, some lengthened out to form tubes 

 through which the sap may pass, still others soft and 

 dividing rapidly. The stem is growing fast in the region 

 where these soft, rapidly dividing cells are formed, for 

 living things grow in size by the increase in the number 

 of their cells. 



Living Things Are Responsive. Then living things are 

 responsive to conditions outside of themselves. A dog 

 comes when you call him, a plant turns toward the source 



