84 



EXPERIMENTS WITH PLANTS 



off the pointed end a little so as to allow a small open- 

 ing for drainage; to prevent the root from growing 

 through this opening, close it wdth a piece of gravel. 

 The spring should be of such a stiffness that an ounce 

 will compress it about an inch. Such a spring is 



easily made by winding a brass 

 wire closely and evenly on a 

 round stick (about one-eighth 

 of an inch in diameter) . Se- 

 cure the tubes with wire (or 

 elastic bands) to a strip of 

 wood, as shown in the figure; 

 pin the seed to this strip and 

 clamp a small piece of wood 

 above for it to press against; 

 the clothes-pin which secures 

 the piece of wood also 

 serves to secure the 

 whole apparatus to a 

 tumbler filled with 

 water. Arrange a strip 

 of cotton, as shown in 

 the figure, to keep the seed moist; take care, however, 

 that the soil in wiiich the root grows is not kept too 

 moist, since in that case it will probably rot. As the 

 root grows it forces the tube downward, compressing 

 the spring; when it comes to a standstill, remove the 

 root (but not the earth) from the inner tube and place 



70. Apparatus for determining the force of 

 growth of a root by means of a spring. 



