CHAPTER XVIII 



DIURNAL MOVEMENTS OF PLANTS 



The most diverse and complicated movements of different 

 organs of plants occur in response to variations of the 

 environment, notably when the external conditions undergo 

 a periodic change. The plant is subjected, day and night, 

 to variation of illumination, to change of temperature, and 

 to change of turgor caused by the difference between the 

 accession of water by the root and loss by the transpiring 

 leaves. The organs of plants are, moreover, subjected to 

 the stimulus of gravity, the effectiveness of which varies 

 with the angle of inclination of the organ to the vertical. 



The plant, as already stated, is affected by many forms 

 of stimulation acting simultaneously. The phenomena of 

 plant-movement have remained obscure on account of the 

 numerous factors which contribute to induce them. This 

 is sufficiently illustrated by the consideration of but two 

 out of the agents which affect the plant — the stimulus of 

 gravity and that of light. Certain organs are highly sen- 

 sitive to geotropic stimulation, while others are but feebly 

 sensitive to it. The stronger reaction may be represented 

 by G and the feeble by ^. In regard to light, there are two 

 distinct classes of reaction : positive phototropism, when the 

 organ turns towards the light ; and negative phototropism, 

 when the organ turns away from it. These reactions when 

 strong will be represented by + L and — L ; when feeble, 

 by + / and — /. 



What will be the resulting effect when a horizontal 

 stem is exposed to combined geotropic and phototropic 



