72 PRAC^TICAL COURSE IN BOTANY 



3. Why arc willows so generally selected for planting along the 

 borders of streams in order to protect the banks from washing? (71.) 



4. Why are the conducting tissues of roots at the center instead of 

 near the surface as in stems? (67, 6.) 



5. Why does corn never grow well in swampy ground ? (74 ; Exp. 46. ) 



6. Why are fleshy roots so much larger in cultivated plants than in 

 wild ones of the same species ? (74, 76.) 



7. When the use of a particular kind of fertilizer causes the leaves 

 of the plants to which it has been applied to turn brown, so that the 

 farmer says they have been " burned " by it, to what cause is the trouble 

 due? (59,72.) 



8. Why do farmers speak of turnips and other root crops as "heavy 

 feeders"? (70,71.) 



9. Which is more exhausting to the soil, a crop of beets, or one of oats ? 

 Onions, or green peas? (See 2, suggestion.) 



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

 times cause it to revive? (Exps. 48, 49.) 



V. DIFFERENT FORMS OF ROOTS 



Material. — Examples of taproots : bean, pea, cotton, maple seedlings, 

 or any kind of very young woody root. Fibrous : any kind of grass or 

 grain. Fleshy : parsnip, turnip, carrot, dahlia, sweet potato. Water : 

 duckweed, pondweed, or a cutting of wandering Jew grown in water. 

 Parasitic : mistletoe, dodder, beech drops. Aerial and adventitious : the 

 aerial roots of old scuppernong vines, climbing roots of ivy and trumpet 

 vine, prop roots from the lower nodes of cornstalks and sugar cane. 



78. Basis of distinction. — Roots vary in form and ex- 

 ternal structure according to their origin, function, and 

 surroundings. In reference to the first, they are classed 

 as primary or secondary ; in regard to the second, as dry or 

 fleshy; while as to surroundings, they may be adapted to 

 either the soil, water, air, or the parasitic habit. Soil roots 

 are the normal form. According to their mode of growth 

 they are either fibrous or axial. 



79. Taproots. — These are the common form of the axial 

 type. Compare the root of any young hardwood cion a 

 year or two old with one of a mature stalk of corn or 

 other grain, and with the roots of seedlings of the same 

 species. Notice the difference in their mode of growth. In 



