THE SPERMAPHYTES. 



249 



FIG. 175. Apparatus to show hydrotropism. 

 (After Sachs.) 



too dry nor too damp. Sachs, the originator of the ex- 



periment, recommends a dark closet, the floor of which 



is occasionally 



watered. If the con- 



ditions are right, 



the radicles, as they 



emerge from the 



lower part of the 



box, will not obey 



geotropism, but will 



grow close to the 



damp sawdust. In 



an atmosphere that 



is too moist, the rad- 



icles obey geotro- 



pism. In obedience 



to the fact illus- 



trated by this ex- 



periment, plants send their roots towards sources of mois- 



ture in the earth. The penetration of wells by the roots 



of trees is a well-known 

 source for the contami- 

 nation of well water. 



31. Study twining 

 movements. 



a. Raise Beans in a 

 flower pot. Set a stake 

 in the pot. Do the seed- 

 lings move in the direc- 

 tion of the stake, or 

 mu st they be placed in 



contact with it before 



they begin to twine 

 around it ? Determine how long it takes for the tip to 

 make a complete revolution around the stake. 



FIG. 176. A, part of a plant of Convoi- 



vulus arvemis; 5, part of a Hop vine. 

 (After Edmonds.) 



