566 PLANT RESPONSE 



that of light, either towards or away from it, Some plants, 

 again, close their leaves or leaflets on the approach of night, 

 in the so-called ' position of sleep ; ' while a further complica- 

 tion arises from the fact that an apparently similar ' sleep ' 

 movement is produced, in these or others, by the action of 

 the noonday sun. 



Light, again, appears in some cases to initiate movement, 

 as was seen in Desmodium at standstill, and in others to arrest 

 it, as is said to happen with the spontaneous movements of 

 Trifoliwn pratense. Certain swarm-spores, moreover, appear 

 to be attracted by light, swimming towards it, with energetic 

 beats of their cilia ; while others, on the contrary, are affected 

 in the opposite manner, and swim away. Or the same 

 specimen may be found to swim, now towards, and again 

 away from, light, swinging backwards and forwards like a 

 pendulum. 



We thus see that not a single responsive effect of light 

 has been observed in the case of plant-organs of which an 

 example directly to the contrary may not be found. For 

 this reason it has appeared hopeless to attempt to unify these 

 phenomena, and this fact has left investigators with little 

 option but to tend towards a ' belief in the individuality of 

 the plant in deciding what shall be the effect on it of external 

 conditions.' ^ 



So far we have been considering only the diversity of the 

 responsive effects which are induced by light in plants. 

 When we come, however, to the further question of the 

 responsive mechanics by which the stimulus of light evokes 

 these movements, we are confronted at the outset of our 

 inquiry by the fact that, as Pfeffer says, ' the precise character 

 of the stimulatory action of light has yet to be determined.' ^ 



Differentiation of responsive movements. — It is cus- 

 tomary, in treating of plant physiology, to draw sharp lines of 

 demarcation between the different classes of movements which 

 are to be attributed to the action of light, ascribing each to 



' Francis Darwin, B.A. Report. 



' Pfeffer, Physiology of Plants, English translation, 1903, vol. ii. p. loi. 



