CHAP. v. LEAVES 31 



in the leaves, with the carbon of the carbonic acid gas 

 absorbed from the air, and from these various elements 

 thus obtained are manufactured, partly in the leaves and 

 partly elsewhere in the plant, all the plant substance. 



The leaves, then, are what we may call absorbing, excreting, 

 and assimilating organs : absorbing, because they take up 

 gases from the atmosphere ; excreting, because they get rid 

 of the superfluous water and oxygen gas ; and assimilating, 

 because they take substances unlike plant substance and 

 manufacture them into plant substance itself, in order to 

 repair the " wear and tear " of the various parts of the plant 

 and in order to furnish material for the formation of new 

 parts. 



XVI. Questions about Leaves. 



1 . What is a leaf ? 



2. What are the parts of a typical leaf? 



3. What is the difference between a simple and a com- 



pound leaf? 



4. What is meant by venation ? 



5. What is a netted-veined leaf? 



6. What is a parallel-veined leaf? 



7. What is the difference between a pinnately and a pal- 



mately compound leaf? 



8. Describe the differences between the two types of 



netted-veined leaves. 



9. Describe the differences between the two types of 



parallel-veined leaves. 



10. When is a leaf said to be decompound? 



11. What three sets of things make up the substance of a 



leaf? 



12. Describe the epidermis. 



13. Describe the green pulp. 



