86 



PHYSIOLOGICAL PROCESSES IN PLANTS 



Conclusion. Show why it is important that grain crops 

 alternate with tubercle-bearing crops. 



Note. A regular order of crops, commonly including some 

 tubercle-bearing crops, is known as crop rotation. 



Observations. 1. Study the following four systems of crop 

 rotation and see if you can justify them. 



2. Which crops are nitrogen gatherers? Which for cash 

 income? 



3. Why is it that tubercle-bearing crops will grow where 

 ordinary grain and garden crops fail ? 



Conclusion. Explain fully just what is meant by crop rota- 

 tion. Why is it of supreme importance to man ? 



Questions 



1. Name some roots of commercial importance. Why are 

 they important? 



2. Why do some plants store up food in their roots ? 



3. What are water roots ? Air roots ? 



4. What are parasitic roots ? Give examples. 



