PLANT PHYSIOLOGY 305 



the nutation of growing tips only becomes apparent through 

 prolonged experiment. 



The other prominent growth movement is that which 

 places organs in proper relations for their work., sending 

 roots into the soil and stems into the air, and directing 

 leaf planes in various ways. For example, in the germina- 

 tion of an ordinary seed, in whatever direction the parts 

 emerge the root curves toward the soil, the stem turns 

 upward, and the cotyledons spread out horizontally. 



The movement of nutation seems to be due largely to 

 internal causes, while the movements which direct organs 

 are due largely to external causes known as stimuli. Some 

 of the prominent responses to stimuli concerned in direct- 

 ing organs are as follows : 



Heliotropism. — In this case the stimulus is light, and 

 under its influence aerial parts are largely directed. Plants 

 growing in a window furnish plain illustration of helio- 

 tropism. In general the stems and petioles curve toward 

 the light, showing positive heliotropism (Fig. 279); the 

 leaf blades are directed at right angles to the rays of light, 

 showing transverse heliotropism ; while if there are hold- 

 fasts or aerial roots they are directed away from the light, 

 showing negative heliotropism. The thallus bodies of ferns, 

 liverworts, etc., are transversely heliotropic, as ordinary 

 leaves, a position best related to chlorophyll work. If the 

 light is too intense, leaves may assume an edgewise or pro- 

 file position, a condition well illustrated by the so-called 

 "compass plants." (See Plant Relations, p. 10.) 



Geofropism. — In this case the stimulus is gravity, and 

 its influence in directing the parts of plants is very great. 

 All upward growing plants, as ordinary stems, some leaves, 

 etc., are negatively geotropic, growing away from the center 

 of gravity. Tap-roots are notable illustrations of positive 

 geofropism, growing toward the source of gravity with con- 

 siderable force. Lateral branches from a main or tap-root, 

 however, are usually transversely geotropic. 



