MECHANICAL USES OF WATER IN THE PLANT 37 



cut for a distance of about 25 mm. from the end, 

 and just beneath the surface. 



8. Describe the position assumed by the severed piece. 

 Illustrate by a diagram, natural size. 



9. From another petiole cut off a portion at least 

 15 cm. long, with the cut surfaces normal to the 

 edges. Record the exact length of the piece in 

 millimeters. 



10. With a scalpel carefully remove a thin strip of 

 outer tissue along the entire length of the piece 

 (or remove a strip of "bark" from a very young 

 woody stem). At once try to replace it. Has it 

 altered in length? If so, describe. Make another 

 similar observation at the end of ten or fifteen 

 minutes. What would you have to do to the 

 strip to make it resume its former length? 



11. Carefully measure the length of the excised strip 

 about fifteen minutes after its removal. Record 

 this measure, and calculate the percentage of change 

 in length. 



12. From another portion of the petiole cut off two 

 strips from opposite sides (or the bark from a 

 portion of some young woody stem). Place one 

 of the excised strips in water, another in a 10 

 per cent, salt solution. 



13. At the end of five or ten minutes compare the 

 lengths of the two strips, (a) with each other, 

 (b) with the portion of the stem from which they 

 were cut. Explain what you observe. 



14. From the preceding studies describe the condition 

 of the tissues in a plant stem. To what is this 

 condition due? 



15. Of what advantage do you think this condition 

 would be to^the plant? 



