LECTURE XXI. 



IRRITABILITY (continued}. 



The Mechanism of the Movements. 



WE have now concluded our description of the move- 

 ments of plants, whether spontaneous or induced, whether 

 performed by mature or by growing organs. In the course 

 of the description the mechanism of the movements has been 

 incidentally alluded to, but now it will be fully discussed, 

 though some portion of this lecture will be devoted to the 

 consideration of the biological significance of the movements 

 and of the peculiar forms of irritability of which they are the 

 expression. 



With regard to the mechanism of the movements of loco- 

 motion of motile organisms, with which we will begin, all 

 that can be said is that whether the movement be amoeboid 

 or ciliary, it depends upon that fundamental property of 

 protoplasm which was mentioned in the first lecture, namely 

 contractility. What contractility really is we do not know; 

 all that we know are the manifestations of it. It must be 

 borne in mind that our conception of this property of proto- 

 plasm, and the very name we give it, is based upon the 

 phenomena presented by the most highly differentiated con- 

 tractile protoplasm, the striated muscular fibre of animals. 

 The fibre, on stimulation, shortens, and at the same time 

 thickens to about an equal extent; hence it is said to 

 contract. But it must not be assumed that the effect of 

 stimulation upon all forms of contractile protoplasm will 

 be precisely this, nor must we limit the term contraction 



