502 Plant Physiology 



probability that positive chemotropism is not a highly 

 developed response. It may occur in roots and pollen- 

 tubes, although the evidence is not entirely convincing; 

 while serious doubt has been thrown upon the existence 

 of positive chemotropism in fungous hyphre. 



305. Nutation. - The tips of growing axes or other 

 plant members are not as a rule extended in a straight 

 line. Instead, they nod here and there or commonly trace 

 an irregular spiral, the projection of which yields a series 

 of more or less circular or elliptical figures. This type of 

 movement is called nutation (circumnutation). It was 

 extensively studied by Darwin, and the main effects to- 

 gether with some of the important relations were clearly 

 set forth at that time. 



The type of curve varies with the growth relations. In 

 stems which are radially symmetrical nutation results 

 from unequal growth in the vertical segments. The 

 effects produced are accounted for by greater growth in 

 each segment successively around the stem. When 

 asymmetry occurs, and especially in flattened or dorsi- 

 ventral organs there is more likelihood that the movement 

 will tend toward narrow ellipses or even the back-and-forth 

 linear type. The extent of the movement depends upon 

 the unevenness and rapidity of growth. It is generally 

 greatest in organs growing rapidly, such as tendrils and 

 climbing shoots, and the whole of the growing region may 

 be involved. Nevertheless, the pronounced nutation of 

 twiners does not begin, as a rule, until after a f ew internodes 

 are produced. Tendrils, likewise, show little nutation 

 during the early stages of growth, and the movement 

 ceases in matured organs. 



