16 



PHYSIOLOGY 



as a moist membrane. A cell-wall is a moist membrane. 

 If the particles of a liquid or a gas come in contact with 

 it they pass into the spaces between the particles of the 



Fio. 8.— A diagram illustrating osmosis. Two substances separated by a mem- 

 brane, A, before ; B, after osmosis has begun. 



cell-wall and get through to the other side. In this way 

 water and carbon dioxide pass into the root and leaf, 

 starch into the potato cell, and substances pass from cell 

 to cell in the bodies of organisms. 



Chemical Action. — How are starch grains made? 

 Water and carbon dioxide enter the plant by a physical 

 process which involves no change in their nature. In 

 the plant they are manufactured into starch, an abso- 

 lutely different substance. This is done by a chemical 

 process. In chemical action two or more simple sub- 

 stances unite to form a complex substance, or a complex 

 substance splits into the simple substances of which it is 

 composed. 



Combustion. — Carbon dioxide is a colorless gas, and 

 as its name indicates it is made of two substances, car- 

 bon and oxygen. Oxygen is a colorless gas and as far as 

 appearance goes it is not to be distinguished from carbon 



