Ch. IV. 9] 



GEOTROPISA[ OF STEM- 



175 



pliototropir^m, they avf growth niovements, as t'xperiniriit 

 proves. If the plant be kept re\-olving upon a chiiostat, how- 

 ever (Fig. 12r, then 

 no such responses can 

 OCCUI-. and the parts 

 continue to grow in the 

 directions they happen 

 to have at the start. 

 Everybod}- knows how 

 stiffly upright are the 

 Fir trees, and hovr 

 remarkably horizontal 

 their branches (Fig. 

 122) : and this is as 

 true on the steepest 

 hillsides as on level 

 ground, showing that 

 the upright position is 

 not in any way deter- 

 mined by the slope of 

 the surface from \^-hich 

 the trees grow. The 

 facts here cited are 

 tAijical, and represent 

 a verT,- -n-idespreacl at- 

 tribute of the higher 

 plants, — that they 

 grow in such maruier 

 as to S"ning their main 

 roots straight doT\ii, 

 their main stems straight up. and their side roots and stems 

 at definite angles to the up-and-down direction. 



The up-and-do'UTi. and the horizontal, directions on the 

 earth's surface are determined by a single factor, viz. gi-a\i- 

 tation, to which, accorchngly. plants show remarkable ad- 

 justments in their gi-oviTh. — a property called geotropism. 



Fig. 120, — A Pelargoaiuni. inverted 

 and kept in the dark : X -j. Xote that the 

 individual leaf blades have taken positions 

 approximating towards horizontal. (Drawn 



froni a photograph.) 



