Bailey, _The Evolution of our Native Fruits_. The Macmillan
Company.
Bailey, _Plant Breeding_. The Macmillan Company.
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Vol. I.
American Book Company.
De Candolle, _Origin of Cultivated Plants_. D. Appleton and
Company.
Duggar, _Plant Physiology_. The Macmillan Company.
Farmers' Bulletins, Nos. 78, 86, 225, 344. U. S. Department
of Agriculture.
Hodge, _Nature Study and Life_, Chaps. X, XX. Ginn and Company.
Kerner (translated by Oliver), _Natural History of Plants_.
Henry Holt and Company, 4 vols. Vol. II, Part 2.
Sargent, _Corn Plants_. Houghton, Mifflin, and Company.
VI. THE ORGANS OF NUTRITION IN PLANTS--THE SOIL AND ITS RELATION TO THE
ROOTS
_Problem.--What a plant takes from the soil and how it gets it._
_(a) What determines the direction of growth of roots?_
_(b) How is the root built?_
_(d) What is in the soil that a root might take out?_
_(e) Why is nitrogen necessary, and how is it obtained?_