CONTENTS. 



Lecture XXXIII. — Mechanical Causes and Effects of the Growth of Cells and 



Organs 563 



Turgescence as a Cause of Growth. — Cause of Turgescence. — Drooping.— Plas- 

 molysis retards Growth. — Tissue-tensions during Growth. — Transverse Tension in 

 Tissues. — Behaviour of an Isolated Prism of Pith. — Cortical Pressure. — Tissue- 

 tension in roots. — Effect of Pressures in Tissues on the form and size of Cells. — 

 Changes in Wood during alterations of Cortical Pressure. — Growth in Thickness of 

 the Branches of Trees. — Tyloses ; Callus. — Internal and External Work by means 

 of Growth. 



PART v.— IrritabiUty. 



Lecture XXXIV. — General Considerations on Irritability 587 



Disproportionality between Stimulus and Effect. — Stimuli, Stimulation, Irritable 

 Structure. — Stimulation in Crystals. — Rigor of Irritable Organs. — Propagation and 

 after-effect of a Stimulus. — Propagation of a Stimulus compared with Physical 

 Processes. — Spontaneous Periodic Movements, — Specific Energy of Irritable Organs. 



Lecture XXXV. — Irritability and Mobility of Protoplasmic Structures . . . 603 

 Swarm-spores and Antherozoids. — Velocity of Swarm-spores. — Rotation. — Influence 

 of Temperature on the Movement of Swarm-spores. — Emulsion-figures. — Action of 

 Light on Swarm-spores. — Influence of the Colour and Intensity of the Light. — 

 Amoebje, Plasmodia.— Circulation of Protoplasm in the Cell. — Circulation and 

 Rotation of Protoplasm. — Movements of Chlorophyll.— Causes of the Move- 

 ments. 



Lecture XXXVI. — Periodic Movements of Foliage-Leaves and Flowers (Sleep- 

 Movements) 623 



Variations in Illumination induce Movements. — Combination of different causes of 

 Movement. — Phototonus and Paratonic Light-stimuli. — Structure of Motile Organs. 

 — Tissue-tension in Motile Organs. — Histology of Motile Organs. — Changes in the 

 Water-contents due to alterations in the Light. — Description of the Daily 

 Periodicity. — Colours of Light. — Mechanics of the Movement. — Lever Action. — 

 Leaves without special Motile Organs. — Opening and Closing of Flowers. — Use 

 of Sleep-movements. 



Lecture XXXVI I.— The Irritability of Mimosa and other Plants . . . -644 

 Description of the Phenomena of Irritability. — Escape of Water on Stimulation.— 

 The Irritable Movement is the consequence of the Expulsion of Water. — Irritable 

 Stamens of the Cynarese. — Cause of the Expulsion of Water from Stimulated Organs. 

 The Movement results from Changes in the Protoplasm and Cell-wall. — The 

 Irritability of Mimosa as means of protection. 



Lecture XXXVI 1 1.— The Revolving of Tendrils and Twining Plants . . -657 

 Organography of Tendrils. — The Coiling of Tendrils. — Mechanics of the Coiling. — 

 Irritability. — Thick and thin Tendrils. — Advantageous properties of Tendrils. — 

 Nutation previous to Coiling. — Behaviour of Revolving Shoots on Supports. — 

 Twining without Supports. — Klinostat-movement of Free-shoots. — Feeble Shoots. 

 — Comparison of Twining-shoots and Tendrils. — Geotropism. 



Lecture XXXIX. — Geotropism and Heliotropism 677 



Geotropism. — Positive and Negative Geotropism. — Nature of the Geotropic Sti- 

 mulus. — Centrifugal Force. — Klinostat. — Form of Geotropic Curvature depends on 

 the Distribution of Growth. — Changes in Form of Shoot-axes during Erection. — 

 Geotropic Curvature of the Nodes of Grasses. — Motile Organs of Leaves. — Down- 

 ward curvatures. — Changes during Geotropic Downward curvatures. — Heliotropism. 

 Demonstration of Heliotropism. — Theory of Heliotropism. — Action of Colours of 

 the Spectram.— Non-cellular Plants. 



