54 INTRODUCTION TO BOTANY 



which is covered with a clear, sticky fluid. When a small in- 

 sect touches one of the sticky knobs, it may be held fast, and 

 in that case the hairs at once begin to close over it. The insect 

 soon dies and usually remains for many days, while the leaf 

 pours out a juice by which the soluble parts of the insect 

 are digested. The liquid containing the digested portions is 

 absorbed by the leaf and contributes an important part of 

 the nourishment of the plant, while the undigested fragments, 

 such as legs and wing cases, remain on the surface of the 

 leaf or may drop off after the hairs let go their hold on the 

 captive insect. The pitcher plants have cylindrical, liquid- 

 containing leaves, with inner hairs so arranged that insects 

 get into the leaves very easily and get out with great diffi- 

 culty, if at all. The leaves of the Venus's-flytrap close 

 so quickly that they sometimes catch insects which may 

 come in contact with them. 



PROBLEMS 



1. Which are greener, leaves which are densely shaded or those 

 which are exposed to the sun? What makes the difference? 



2. Will alcohol dissolve the chlorophyll more quickly from densely 

 shaded or from openly exposed leaves? Why? 



3. Under what conditions could a field of corn lose water to the soil 

 instead of securing water from the soil? 



4. What would be the effect upon animal life if green plants should 

 manufacture no more food than they consume in their own growth ? 



5. Leaves often give a strong starch reaction in the late afternoon 

 and none in the early morning. Can you account for this? 



6. It was found by some students that a corn plant elongates about 

 four times as much between 6 P.M. and 6 A.M. as between 6 A.M. and 

 6 P.M. Can you account for this fact ? 



7. Fields of grain are often more or less wilted at noon on hot 

 summer days and regain their turgor at night even when no rain has 

 fallen. Can you account for this fact? 



8. In trying to kill plants in the lawn or trees in the cultivated 

 fields people sometimes put salt iipon the roots or stumps. What is the 

 botanical explanation of this practice ? 



