SENSITIVITY AND MOVEMENT 243 



of the mysterious performances that take place in them 

 when they are exposed to sunlight. 



SENSITIVITY AND MOVEMENT 



We have seen that before the end of the nineteenth 

 century it had come to be generally accepted that all 

 organisms, whether plant or animal, were sensitive to 

 external stimuli and responded in various ways, visible 

 or invisible. It was also recognised that it was the proto- 

 plasm that possessed the capacity for appreciating or 

 perceiving the stimulus, but although this power was 

 obviously associated in animals with certain differentiated 

 sense organs, the existence of similar structures in plants 

 was more difficult to establish. It is only within recent 

 years that special sensory cells or tissues have been postu- 

 lated for plants. Then again the sensory organs in animals 

 were well known to be continuous with definite tracts 

 of afferent nerve tissue which transmitted the excitations 

 induced by the stimuli to nerve centres, and thence by a 

 system of motor or efferent nerves to the seats of response, 

 i.e. of movement, secretion, or what not. Nerves and 

 nerve centres in the plant seemed, however, to be con- 

 spicuous by their absence. Finally, although plants had 

 undoubtedly the power of movement, to some limited 

 extent at least, the special contractile tissues so apparent 

 in the animal could not be identified in the motile organs 

 of plants. 



So far as sense organs are concerned we are justified 

 in saying that, as a result of the investigations of Pfeffer, 

 Haberlandt, Borzi, and others, certain sensitive organs, 

 such as tendrils, possess what Haberlandt calls " tactile 

 pits," " tactile papillae," hairs or bristles or " stimu- 

 lators." These latter organs are specially noticeable 

 in carnivorous plants, such as Dionaea, and in various 

 genera of orchids, where they are associated with intricate 

 movements connected with cross pollination. 



