LABORATORY AND FIELD WORK 419 



5. Find as many as possible of the following special leaf struc- 

 tures and determine the special use of each to the plant : 



a. Hairs or scales. 



b. Layers of cutin or wax. 



c. Disagreeably smelling, or poisonous, substances. 



d. Sharp teeth, or cutting edges. 



e. Small, thick leaves. 



6. Find as many leaves as possible that have taken on some of 

 the following special forms : thorns or spines ; tendrils ; bracts ; 

 scales ; forms adapted to catch insects. 



7. Study and draw a cross section of a leaf of EngUsh ivy, nas- 

 turtium, or sunflower, noticing the following tissues : 



a. Upper epidermis ; how many layers of cells ? Are air-pores 



present ? 



b. One or more layers of closely packed cells just below the upper 



epidermis. What is their shape ? 



c. The spongy part of the leaf, containing air spaces. What are the 



shapes of the cells ? 



d. Veins. To what structures in the stem do they correspond ? 



e. The lower epidermis. Compare with the upper epidermis. 



8. Boil some freshly picked nasturtium leaves for a few minutes, 

 then soak them in hot alcohol for half an hour. What happens to 

 the chlorophyl ? Now place them for a few minutes in a solution 

 of iodine. What color do the leaves take on? What does this 

 color change teach you? 



9. Pin two small pill boxes or circular disks of cork on opposite 

 sides of a leaf so as to exclude the light from part of the leaf (the 

 leaf being still attached to the plant) ; leave the plant in bright 

 light for a day or two. Then cut off the leaf and treat it as you 

 did the leaves in the previous experiment. Compare the results 

 of the two experiments. How do you explain the difference? 



10. Obtain a begonia or some other plant whose leaves have air- 

 pores only on their under surfaces. Coat the under surfaces of 

 some of the leaves with vaseline and leave the plant in bright light 

 for a day or two. Then remove the vaseline with benzine and 

 treat the leaves as in the two preceding experiments. Explain 

 your results. 



