CONTENTS xiii 



Page 

 —Foliage leaves— Axial parts: hypocotyls, internodes, and flower 

 stalks — Distribution of growth — Growth substances in relation to 

 the distribution of growth — Roots— Lower plants— Animal 

 cells — Plant tissue culture — Growth substance in relation to cell 

 division. 



The mechanism of action of growth substances 103 



Growth substance and the cell wall— Cell wall extensibility- 

 Methods for measuring extensibihty — Hypotheses on the method 

 of action: elasticity, plasticity, active growth — Discussion of 

 hypotheses: growth substances and elastic extensibility, growth 

 substances and plastic extensibility, growth substances and intus- 

 susception — Acid-growth reaction and the growth hormone — 

 Inhibition of growth in roots. 



Summary 115 



CHAPTER VII 



The Significance of Growth Substances for Other Phenomena 117 

 Bud development — Tumor formation — Stomatal movement — 

 Respiration — Cambial activity — Nastic movements — Root forma- 

 tion — Miscellaneous. 

 Summary 129 



CHAPTER VIII 



The Significance of Growth Substances for Phototropism . . . 131 

 General discussion of phototropism with special reference to the 



Avena coleoptile 131 



Stimulation and response — The light gradient — Distribution of 

 sensitivity to light — Conduction of the stimulus — The quantity- 

 of-stimulus principle — Positive and negative curv^atures — The 

 primary positively phototropic curvature — The Blaauw theory — 

 Evidence supporting the Blaauw theory — Evidence opposing the 

 Blaauw theorj^ — Conclusions in regard to the Blaauw theory — The 

 growth-substance explanation— The relation of growth substance 

 to phototropism — The question of wound substances — Origin 

 of the unequal distribution of growth substance — Contrasting 

 theories of Boysen Jensen and Paal — Purdy's theory — The Went 

 theory — Theories of Beyer, Cholodny, DuBuy and Nuernbergk 

 — The displacement of growth substance — Growth-substance 

 transfer and electrical potential — Conclusions in regard to the 

 growth-substance explanation — Other theories on phototropism. 



Dicotyledonous stems 163 



Distribution of phototropic sensitivity — Transmission of photo- 

 tropic stimulus — Growth substance and phototropism in seedhng 

 axes. 



The Phycomyces sporangiophore 166 



Summary 167 



