THE ROOT 89

25. In what region of the root does the greatest elongation
occur? How may it be proved that the curvatures in
response to stimuli (gravity, light, moisture, contact)
also occur in this part?
26. How do root hairs absorb water? Describe one experi-
ment in illustration.
27. Under what conditions will root-hairs be most and under
what conditions least abundantly developed?
28. What is root-pressure? How is it set up, and how can it
be demonstrated and roughly measured?
29. What peculiarity may be noted in the roots of the sweet
pea and other plants of the same family?
30. What are the commonest nitrogenous manures?
31. Recent research shows that chlorophyll contains
C.O.H.N. and magnesium. Indicate the source from
which, the form in which, and the organ through
which the plant obtains each of these elements.
32. What elements constitute the substance of a green plant?
Through what organs, from what source, and in what
form is each of these elements absorbed?
33. How is the root-cap formed? Why does it not lengthen
indefinitely?
34. Why is the growing-point of the root protected by a cap
while that of the stem is not so proected?
35. What structures protect the growing-point of the stem?
What need is there for such protection?
36. What is a growing-point? Compare fully the growing
points of root and stem.
37. Give the derivation of and meaning of the term "geo-
tropism." What is the cause of geotropic phenomena?
38. Give one example of a plant in which some of the roots
are negatively geotropic. Explain how this peculiarity
serves to adapt the plant to its environment. Suggest
circumstances under which the stem might become
positively geotropic.
39. What relation exists between the positive geotropy of the
root and the plant's need for water?
40. Which exerts the stronger pull on the root, water or
gravity? Describe experiments that will illustrate your
answer, and explain the advantage derived by the
plant from the behaviour of its root when attracted in
different directions by water and gravity respectively.