60 LIFE ON THE FARM. 



HOW A LEAF WORKS. 



There is busy work, and there are complex 

 changes going on in leaves during the day while 

 the sun shines. As soon as compounds are formed 

 in the leaves, they are carried away by the circula- 

 tion of the sap to all parts of the plant, to be built 

 into tissue. Starch is not soluble, but sugar is; so 

 the starch is transformed into sugar and carried by 

 the sap to the places where most needed, and there 

 changed back again to starch, cellulose, fat, gum, 

 or some other similar compound. At night, of 

 course, when there is no light, starch-building 

 ceases, and the leaves stop taking in carbonic acid 

 gas; but, as with animals, vital activity never 

 entirely ceases so long as there is any life. During 

 the night rest of plants there is a process, though 

 less active, going on in the leaves similar to the 

 breathing of animals, In which oxygen is taken in 

 and carbonic acid gas given off. 



It may be well to add here that there can be no 

 life without a peculiar substance called protoplasm. 

 It is called by some " the physical basis of life." 

 Protoplasm is a watery, transparent, jelly-like sub- 

 stance, very plastic, and always in motion. It is 

 composed of carbon, hydrogen, oxygen, nitrogen, 

 and sulphur. It is the basis of animal life as well 

 as of plant life. Plants build organic substances 

 out of inorganic material, while animals consume 

 or destroy such substances. From this it is very 



