SPECIAL SENSES 



power to vary from a straight line. If a seed 

 in germinating lies in such a position that the 

 roots point upwards and the stem down- 

 wards, some method is needed by which the 

 plantlet may readjust itself, either by turning 

 over bodily, or changing the direction of its 



Fig. 1. — A clinostat. Germinating seeds are pinned on the 

 cork disc, and are kept moist by dipping into water as they 

 revolve. One revolution is made in about twenty minutes. 

 Under these conditions the primary root and stem grow 

 straight in whatever position they are fastened. 



growing parts. As every one knows, the lat- 

 ter alternative is adopted, and the roots bend 

 -down and penetrate the earth, while the stem 

 bends up and lifts its foliage into the air. It 

 is so apparently a matter of course that 

 stems grow up and roots grow down, that 

 we may never have given a thought to an 

 explanation of the process. Even botanists 

 have only recently felt the full necessity for 

 an explanation of the fact, as it has been less 

 than tw^o decades since Vochting announced 

 his theory of rectipetality, or the inherent 



