CHAPTER XLI 



FUNDAMENTAL RESPONSIVE ACTION OF PLANT-TISSUES 

 TO STIMULUS OF LIGHT 



Diversity of movements induced by light — Differentiation of responsive movements 

 — Action of light on tissues in sub-tonic conditions— Effect of light on pulvinated 

 organs — Effect of diffuse stimulation of light on non-growing radial organs- — 

 Retarding effect of light on longitudinal growth— Phenomenon of oscillation 

 under long-continued stimulation — Similarity of responsive reaction under 

 light and under other forms of stimulation. 



There is perhaps no other phenomenon in the plant-world 

 which is at once so striking and so universal as that of the 

 response which is evoked from plants by the stimulus of 

 light. Under this influence the plant as a whole, and every 

 part of it, is tremulous. Not only does the growing stem 

 curve towards or away from the incident rays, but every 

 leaf under their action is thrown into a state of periodic daily 

 rhythm. By the absorption of light, again, the plant is 

 energised. Thus the two factors, of external stimulus and 

 internal energy, whose manifestations are so opposite in 

 character, are brought into play 



Diversity of movements induced by light. —Light, as a 

 form of external stimulus, evokes responsive movements, which 

 appear to be extremely diverse in their nature. Radial organs, 

 for example, such as stems, under certain conditions, direct 

 themselves towards the light, and under others, again, away 

 from it. Leaves, however, behave quite differently. These 

 are said to possess the peculiarity of placing themselves with 

 their surfaces at right angles to the light ; but even this 

 is not a property universally exhibited, for there are certain 

 leaves which place themselves in a direction coinciding with 



