THE MOVEMEXTS OF PLANTS. I97 



15° to 20". The flowers of the white \vater-Hl\" (XvniphKaj 

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

 B\' marking upon their lea\"es a series of equidistant parallel 

 lines with Chinese ink, and subsequent!)- measuring the dis- 

 tances to which the)' have been spread, all such movemenls 

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

 outer or inner surfaces, respeclivelv. 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 lieat ra)s. 



■287. (r) Geotropism. — Ceotropisui is the state of a plant 

 or an organ when it is irritable to tire action of gra\il\-. 

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

 jilain 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 relati\-ely different direction. 

 The reaction to this stinuilus 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 ]ilants with slender 

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

 weight of the ])arts. True geotropic cur"\-atures are brought 

 about by acceleration of the growth of the irritable cells, 

 and the curvatures produced mav 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 \'ertical plane, so that 

 thev are successi\'eh" subjected to the action of gra\"ity in 

 relati\-elv different directions, it will be seen that while their 

 members grow in nearly straight lines, the direction assumed 

 bv the stems and roots is quite as frequently abnormal as 

 normal, because the effect of gra\'ity 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 



