THE MOVEMENTS OF PLANTS. 1 97 



15 to 20 . The flowers of the white water-lily (Nymphsea) 

 and of the dandelion open in sunlight and close in shade. 

 By marking upon their leaves a series of equidistant parallel 

 lines with Chinese ink, and subsequently measuring the dis- 

 tances to which they have been spread, all such movements 

 can be clearly shown to be due to accelerated growth of the 

 outer or inner surfaces, respectively. The protection of the 

 flower parts or the proper discharge of the functions is secured 

 by these movements, which must not be confounded with 

 those due to the direction of light or heat rays. 



"287. (c) Geotropism. — Geotropism is the state of a plant 

 or an organ when it is irritable to the action of gravity. 

 Since gravity is exerted always in the same direction, it is 

 plain that reactions to this force cannot be studied, as in the 

 case of light, by altering the absolute direction in which 

 gravity acts, but only by so changing the position of the 

 plant that the force acts in a relatively different direction. 

 The reaction to this stimulus and the fixed gravity position 

 must not be confused with the simple effect produced by the 

 weight of the parts concerned. Such effects are to be seen 

 in the downward bending of some plants with slender 

 branches, or the curvature of the flower or fruit stalks by the 

 weight of the parts. True geotropic curvatures are brought 

 about by acceleration of the growth of the irritable cells, 

 and the curvatures produced may even be contrary to the 

 direction of the force. If seedlings be grown in boxes upon 

 the rim of a wheel rotating slowly in a vertical plane, so that 

 they are successively subjected to the action of gravity in 

 relatively different directions, it will be seen that while their 

 members grow in nearly straight lines, the direction assumed 

 by the stems and roots is quite as frequently abnormal as 

 normal, because the effect of gravity which normally deter- 

 mines the- direction of growth of these axes is neutralized, 

 since it now acts upon them from a new direction at each 



