16 THE USES AND ORIGIN 



this, two and a half minutes were needed for the hair, or 

 'tentacle' as it has been termed, to move through an 

 angle of 45°. As a rule, the rate of movement is even 

 much slower. The stimulus which provokes movement 

 may come to the base of a marginal tentacle either from 

 its own sensitive tip, or by radiation from some of the 

 shorter hair-like projections near the centre of the leaf 

 whenever their terminal glands have been excited by con- 

 tact with a foreign body. 



The transmission of a stimulus from one of the glands 

 tipping a marginal tentacle in the Sun-dew, to certain cells 

 near its base, though consisting only of molecular move- 

 ments, becomes in a manner visible, owing to the fact that 

 during its passage the protoplasm within the cells of the 

 tentacle undergoes certain obvious changes. Protoplasm 

 previously in a state of uniform diffusion throughout each 

 cell, is caused to aggregate into masses of different size 

 and shape as the invisible wave of molecular movement 

 passes through it. This ' aggregation ' is therefoi-e a 

 visible sign marking the passage of the invisible stimulus. 

 And as Darwin points out, the phenomenon is analogous 

 in certain respects to that which occurs when, after 

 stimulus, an invisible molecular change traverses a nerve 

 in an animal organism.* 



The same observer has discovered that the chief delay 

 in the transmission of the stimulus along the tentacle 

 of the Sun-dew is caused by its having to traverse tho 

 successive cell- walls which lie across its path. At each 

 barrier of this kind an appreciable retardation occurs, as is 

 evidenced by the interval that elapses between the com- 

 pleted aggregation in one cell and the commencement of 

 the process in the protoplasm of that which stands next 



"Insectivorous Plants." 1875, p. (33. 



