THE NEEDS OF THE PLANT 119 



their food from the ground. If the ground is not rich 

 the plants will not grow well because they will not be 

 able to obtain enough food. Farmers add plant-food to 

 the ground, where the ground is not rich enough. Any 

 plant-food which is added to the ground is called a fertil- 

 izer. All plant-food must be such as will dissolve 

 in water after it is put upon the ground. 



Thus plants need light, heat, air, water, and food. Do 

 you think that you need much more in order to live? 



Review Questions, 22. 



1. Why do plants grow best in summer? Give sev- 

 eral reasons. Why is the glass of greenhouses some- 

 times covered with white paint? 



2. Why is it bad for a plant to soak the soil with 

 water so that the water stands on the surface of the soil 

 for several hours? 



3. Why does water go into the soil? 



4. If you were buying a farm how would you decide 

 whether the soil was good? 



5. Why do the plants soon die in Experiment 5fi? 

 Which plants live the longer, those from large seeds or 

 from small seeds? 



6. What is the advantage of germination tests? 



7. Why is the sand watered in Experiment 57 by 

 pouring the water upon a piece of paper? 



8. Why is it necessary to know the proper depth of 

 planting? Do you think that a difference of a quarter of 

 an inch would do any harm to a seed which should be 

 planted two inches deep? 



9. Name and discuss all the needs of a plant. 



