Xil CONTENTS, 



•PAGE 



Sect. 19. Effect of Temperature on Growth 74^ 



„ 20. Action of Light on Growth. — Heliotropism 752 



„ 21. Influence of Gravitation on Growth. — Geotropism 758 



„ 22. Unequal Growth 7^5 



„ 23. Torsion 77° 



„ 24. The Twining of GHmbing Plants 772 



„ 25. The Twining of Tendrils 775 



CHAPTER V. 



Periodic Movements of the Mature Parts of Plants and Movements 

 dependent on Irritation. 



Sect. 26. Definition 782 



„ 27. Review of the Phenomena connected with Periodically Motile and Irritable 



Parts of Plants 783 



„ 28. Mobile and Immobile Condition of the Motile Parts of Plants . . . 788 



„ 29. Mechanism of the Movements 792 



CHAPTER VI. 



The Phenomena of Sexual Reproduction. 



Sect. 30. The Essential Element in the process of Sexual Reproduction . . . 801 

 „ 31. Influence of the origin of the Reproductive Cells on the product of 



Fertilisation 807 



„ 32. Hybridisation 816 



CHAPTER VII. 



The Origin of Species. 



Sect. 33. Origin of Varieties . 822 



„ 34. Accumulation of new characters in the Reproduction of Varieties . . 826 



„ 35. Causes of the Progressive Development of Varieties 830 



„ 36. Relationship of the Morphological Nature of the Organ to its adaptation to 



the Conditions of Plant-life 835 



„ 37. The Theory of Descent 842 



Appendix . . 847 



Index 849 



