CH. IV, 9] 



GEOTROPISM OF STEMS 



175 



phototropism, they are growth movements, as experiment 

 proves. If the plant be kept revolving upon a clinostat, how- 

 ever (Fig. 121), then 

 no such responses can 

 occur, and the parts 

 continue to grow in the 

 directions they happen 

 to have at the start. 

 Everybody knows how 

 stiffly upright are the 

 Fir trees, and how 

 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 which 

 the trees grow. The 

 facts here cited are 

 typical, and represent 

 a very widespread at- 

 tribute of the higher 

 plants, that they 

 grow in such manner 

 as to swing their main 

 roots straight down, 

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

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



The up-and-down, and the horizontal, directions on the 

 earth's surface are determined by a single factor, viz. gravi- 

 tation, to which, accordingly, plants show remarkable ad- 

 justments in their growth, a property called GEOTROPISM. 



FIG. 120. A Pelargonium, inverted 

 and kept in the dark ; X $. Note that the 

 individual leaf blades have taken positions 

 approximating towards horizontal. (Drawn 

 from a photograph.) 



