Chap. II. INFLECTION DIRECTLY CAUSED. 33 



to transmit a motor impulse througliout tlie whole 

 length of the j^edicel, consisting of about twenty cells, 

 to near its base, causing this part to bend, and the 

 tentacle to sweep through an angle of above 180°. 

 That the contents of the cells of the glands, and after- 

 wards those of the pedicels, are affected in a plainly- 

 visible manner by the pressure of minute particles, we 

 shall have abundant evidence when we treat of the 

 aggregation of protoplasm. But the case is much more 

 remarkable than as yet stated ; for the particles are sup- 

 ported by the viscid and dense secretion ; nevertheless, 

 even smaller ones than those of which the measure- 

 ments have been given, when brought by an insensibly 

 slow movement, through the means above specified, into 

 contact with the surface of a gland, act. on it, and the 

 tentacle bends. The pressure exerted by the particle 

 of hair, weighing only -y-gJ^-o of a grain and supported 

 by a dense fluid, must have been inconceivably slight. 

 We may conjecture that it could hardly have equalled 

 the millionth of a grain; and we shall hereafter see 

 that far less than the millionth of a grain of phos- 

 phate of ammonia in solution, when absorbed by a 

 gland, acts on it and induces movement. A bit of 

 hair, -^ of an inch in length, and therefore much 

 larger than those used in the above experiments, was 

 not perceived when placed on my tongue ; and it is 

 extremely doubtful whether any nerve in the human 

 body, even if in an inflamed condition, would be in 

 any way affected by such a particle supported in a 

 dense fluid, and slowly brought into contact with the 

 nerve. Yet the cells of the glands of Drosera are thus 

 excited to transmit a motor impulse to a distant point, 

 inducing movement. It appears to me that hardly 

 any more remarkable fact than this has been observed 

 in the vegetable kingdom. 



