144 EXPERIMENTS WITH PLANTS 



planted, as shown in Fig. 82. Water it until the roots 

 have grown down along the litmus paper; then cease 

 watering. In the course of a day or so the result 

 should be plainly visible. This method, while sim- 

 pler, is much less beautiful than the preceding. 



Another way for testing the roots for excretion of 

 acids is to obtain a piece of polished marble, on which 

 is placed a layer of moist sand about an inch deep, 

 in which seeds are placed. As their roots come in 

 contact with the marble they should, if they excrete 

 enough acid and the time is sufficient (two to six 

 weeks), produce a slight etching of the polished sur- 

 face; if such etching occurs it may be made more 

 striking to the eye by rubbing powdered graphite 

 (obtained by scraping the lead of a pencil) on the mar- 

 ble. It is often more convenient to use a seedling with 

 a stout root (Bean, Scarlet Runner or Pea), which is 

 laid directly on the polished marble, covered with wet 

 filter paper which dips into a dish of water and is 

 pressed down by laying a piece of glass over it. In 

 place of marble we may use a mixture of equal parts of 

 plaster of Paris (previously well heated) and whiting 

 (or fine marble dust) rubbed up together in water 

 and then poured out on a piece of glass and allowed 

 to harden: if this be carefully removed from the 

 glass the surface will have sufficient polish for the 

 purpose. 



Roots are able to decompose even the hardest rocks, 



