40 MOVEMENTS OF PLANTS 



along the line so perceived is a much greater miracle 

 than a leaf that closes its leaflets when burnt or cut 

 or shaken. 



I shall show that certain parts of the plant have 

 the special quality of the perception of gravitation, 

 but we are at present ignorant of how the act of 

 perception is effected. We know something of the 

 machinery of hearing or vision in animals, but in 

 plants we can only guess that when a cell is placed 

 horizontally a resulting change of pressure on the 

 protoplasm produces that loss of equilibrium which 

 is translated into curvature. It is, however, 

 probable that Nemec and Haberlandt are right, 

 and that the stimulus depends on the pressure of 

 solid particles, e.g. starch-grains, on the protoplasm.^ 



The use of this gravitational sensitiveness is 

 clear enough. It is to the pine tree what a plumb- 

 line is to the builder, for neither plant nor man can 

 build high unless he builds straight. A man has a 

 general perception of the verticalness of his body 

 and of surrounding objects, but he does not trust 

 to this sense in placing brick on brick to make a 

 house. He uses a plumb-line which tells him 

 through his eye the precise line along which he must 

 pile his bricks. The tree has also to pile one over 

 another the cells or chambers in which its proto- 

 plasmic body lives, and this too must be done along 

 a vertical line ; but the plant is guided by the 

 sensitiveness to gravity of which I have spoken. 



It must be clearly understood that gravity does 

 not act directly on the growth of plants. It does 



^ See their papers in the Deuisch Bot. Ges., 1900, and my 

 summary in a paper read before the British Association, 1905. ^ 



