Xii CONTENTS 



Page 



17. Findings and interpretations of Kastens 99 



18. Findings and interpretations of MacDougal and of Arndt. . . 101 



19. Ck)mments on evidence supposed to prove backward flow 



through the xylem 104 



Summary 106 



CHAPTER V 



A Comparison of Criteria and Methods Used to Determine the 



Tissues of Transport 109 



20. The movement of solutes introduced through cut tissues . . . 109 



21. The movement of solutes introduced through uncut roots . . .112 



22. Tests for the deposition of solutes introduced through cut or 



uncut roots 115 



23. The occurrence of a substance in a tissue as a criterion of its 



movement through that tissue 116 



24. Evidence from movement observed after cutting or blocking 



one and then another of the possible conducting channels . 124 



25. Possible injury to the xylem when the phloem is cut 125 



Summary 131 



CHAPTER VI 



The Method OF Movement through the Phloem 133 



26. Peculiarities of the system 133 



27. Older hypotheses and suggestions 136 



28. The hypothesis of Miinch 143 



29. Points favoring the hypothesis of Miinch 147 



30. Weaknesses in the hypothesis of Miinch 149 



1. Does not allow for simultaneous movement in both direc- 



tions 149 



2. Pressure gradients inadequate 150 



3. Pressure gradients in wrong direction 152 



4. Fails to account for movement of specific substances ... 159 



5. Removal of water not always accounted for 160 



6. Cut stems placed in water fail to empty 160 



7. Does not account for stoppage by local chilling or anesthetics 162 



8. Tends to carry protoplasm and not allow for its return . . 162 



31. The hypothesis of Crafts 164 



32. Weakness in the hypothesis of Crafts 166 



33. Hypothesis accounting for transport by moving protoplasm . 174 



34. Weaknesses in the hypothesis accounting for transport by 



protoplasmic streaming and diffusion 178 



a. Inadequacy of streaming 178 



h. Streaming not demonstrated in sieve tubes 185 



c. Streaming not found to accelerate movement of lithium or 



caffein 185 



d. Simultaneous movement in both directions not demon- 



strated 186 



e. Movement possibly not determined by diffusion gradients 187 



35. The relation of concentration gradients to transport 188 



