xiv CONTENTS 



PAGE 



II. Tactile Sense-organs - 574 



A. Tactile Pits 575 



B. Tactile Papillae 580 



C. Tactile Hairs and Bristles ------ 586 



JTT. Gravitational Sense-organs (Statocysts) - 595 



A. General Considerations ------- 595 



B. The Statolith-apparatus in Roots ----- 599 



C. The Statocysts of Stems and Leaves ----- 603 



D. Experimental Observations regarding the Dependence of 



Geotropic Sensitiveness upon the Presence of Statoliths 609 



IV. Optical Sense-organs 613 



A. Perception of Light by Parallel otropic Organs - - 613 



B. The Optical Sense-organs of Foliage-leaves - - - 615 



1. Papillose Epidermal Cells 616 



2. Light-perception by Epidermal Cells with Flat Outer 



Walls 624 



3. Localised Organs of Light-perception (Ocelli, etc.) - 626 



4. Experimental Observations on the Light -perceiving 



Function of the Upper Epidermis of Leaves - 628 



C. Eye-spots - - - 630 



CHAPTER XIII. 

 THE STIMULUS-TRANSMITTING SYSTEM. 



I. General Considerations ...... 632 



II. Protoplasmic Transmission of Stimuli - 634 



A. Intercellular Transmission ------ 635 



B. Intracellular Transmission - - - - - - 638 



III. Transmission of Stimuli in Mimosa pudica - - 641 



CHAPTER XIV. 



SECONDARY GROWTH IN THICKNESS OF STEMS AND ROOTS. 



A. Normal Secondary Thickening 647 



I. General Considerations ------- 647 



IT. Secondary Growth in Thickness in Gymnospermr and 



Dicotyledons --------- 649 



A. The Cambium ........ 849 



