MOLECULAR MOVEMENTS. 3 



meeting together as do the two halves of a sheet of 

 paper when folded. This movement may be excited in 

 any individual stalk, most easily by touching or softly 

 rubbing the under surface of that part of it which is 

 immediately attached to the branch. At this point the 

 leaf-stalk is attached to the branch by a lump-like thick- 

 ening or node. Similar nodes occur at the bases both of 

 the secondary leaf-stalks and of the leaflets. If one of 

 these nodes is cut through, a bundle of iibres is observ- 

 able in the centre, round which there is a layer of cells, 

 very full of sap, the walls of which are thicker on the 

 upper, thinner on the lower side. Between the cells 

 are spaces filled with air. Now, it can be shown that 

 the bending movement is due to the fact that part of 

 the fluid matter passes out of the cells into the inter- 

 mediate spaces, so that the cellular tissue becomes weaker 

 and less able to support the stalk. 



Motion of this sort is, however, very different from 

 the motion peculiar to animals, in that in the latter, 

 as we shall presently see, it serves to counteract the 

 pressure of opposed weights ; while in the Mimosa the 

 pressure of the leaf-stalk is downward w^hen the under 

 side of the node becomes slack. Before, however, w^e 

 examine minutely the motion peculiar to animals, men- 

 tion must be made of certain other phenomena of 

 motion which occur partly in the vegetable, partly in 

 the animal world, but which can scarcely be observed 

 without the aid of the microscope, as the efficient forces 

 in these cases are too slight to produce perceptible 

 movements of the larger parts of the mass. 



3. Among these forms of motion we do not include 

 the so-called ^molecular, or Brownian movements, to 

 which the celebrated English botanist Brown first called 



