174 



A TEXTBOOK OF BOTANY [On. IV, 9 



9. THE GEOTROPISM OF STEMS 



When seeds are germinating, it is always found that no 

 matter in what positions they happen to lie, the sprouting 

 roots grow over to point downwards, while the new stems as 

 regularly grow upwards (Fig. 119). If, while these parts 

 are still young, the germinating embryos are overturned, the 

 roots and stems grow around into their former directions. 



FIG. 119. Grains of Corn, fastened to vertical corks, with the root 

 ends all pointing centerwards, and there germinated ; X |. The cork on 

 the left was kept fixed in this position, but that on the right was kept re- 

 volving, clockwise, on the clinostat (of Fig. 121). Since the roots grow much 

 faster than the shoots, the latter were added to the drawings after the 

 former. Note' that on the fixed cork the roots point downward and the 

 shoots upward, while on the revolving cork both roots and shoots continue 

 the general directions in which they happen to start. (Drawn from photo- 

 graphs.) 



Later, the side stems and roots as they develop assume 

 positions which are horizontal, or nearly so ; and they like- 

 wise, if overturned, grow again into those directions. If a 

 young plant in a flower pot be laid on its side, its tip will 

 be found next day to have grown around into the upright 

 position ; and if the pot be completely inverted, then a day 

 later the tip will again be found upright, now in exact re- 

 verse of its original position in relation to the pot (Fig. 120). 

 These adjustments occur in darkness as well as in light, which 

 shows that they are not photo tropic (page 54), though like 



