MOVEMENT OF WATER THROUGH THE STEM 165 



air bubbles in the cavities of the stem, in conjunction with 

 various other causes, have been called in to explain the 

 phenomenon, but so far as our knowledge goes at present 

 none of them seems to account for it satisfactorily. 



PRACTICAL QUESTIONS 



1. In pruning, why should the cutting be confined as far as possible 

 to young shoots? 



2. Why should vertical shoots be cut off obliquely? 



3. Why should pruning not be done in wet weather? 



4. Why will a leafy shoot heal more quickly than a bare one? 

 (24, 25, 26, 200.) 



5 . Why does a transverse cut heal more slowly than a vertical one ? 

 (23i> 232-) 



6. Why does a ragged cut heal less readily than a smooth one? 



7. Why does the formation of wood proceed more rapidly as the 

 amount of transpiration is increased ? (226.) 



8. Why do nurserymen sometimes split the cortex of young trees 

 in summer to promote the formation of wood? (219.) 



9. What is the advantage of scraping the stems of trees? 



10. Explain the frothy exudations that often appear at the cut ends 

 of firewood, and the singing noise that accompanies it. (215, 224.) 



11. What advantage is it to high climbing plants, like grape and 

 trumpet vine {Tecoina), to have such large ducts? (214, 215, 221.) 



12. Why is the process of layering more apt to be successful if the 

 shoot is bent or twisted at the point where it is desired to make it root? 



13. Why do oranges become dry and spongy if allowed to hang on 

 the tree too long? (215, 231, 232.) 



14. Why will corn and fodder be so much richer in nourishment if, 

 instead of pulling the fodder when it is mature and leaving the ears to 

 ripen in the field, we cut down the whole stalk and allow both fodder 

 and grain to mature upon it? (215, 231, 233.) 



15. Why will inserting the end of a wilted twig in warm water some- 

 times cause it to revive? (229.) 



16. Why should we protect the south side rather than the north 

 side of tree trunks in winter? 



17. Why does cotton run all to weed in very wet weather? 



18. Why in pruning a branch is it best to make the cut just above a 

 bud? 



19. Why is the rim of new bark or callus that forms on the upper 

 side of a horizontal wound thicker than that on the lower side? (231.) 



20. Why is it that the medicinal or other special properties of 

 plants are found mostly in the leaves and bark, or parts immediately 

 under the bark? (215, 220.) 



