218 COLLEGE BOTANY 



twisting of teaidrils of such plants as the grape are good illus- 

 trations of the response of the plant to a contact or touch stimu- 

 lus. The tip of the plant or tendril swings^ in a more or less 

 regular circle (nutation) until it comes in contact with some 

 object. The irritability of the part causes, it to respond to this 

 stimulus by twining or twisting. 



Plant Movements. — It is well known that growing plants 

 have the power of making certain well-defined movements, which 

 are no doubt the result of some one or more of the. various irri- 

 tants or environmental agencies. In some plants (Desmodium 

 gyrans) there is a constant rising and falling of the leaflets. 

 The mature stamens of the barberry fly upward when touched, 

 and the pollen is expelled at the same time; since insects are 

 constantly coming in contact with the flowers this movement 

 aids in pollination. The tentacles on the leaves of the sundew 

 (Drosera) move when touched and capture many insects which 

 come in contact with their viscid tips. The leaves of manj plants 

 move under the influence of light and darkness and also under 

 the influence of temperature. 



Tropism. — This term is used to apply to certain curvature 

 movements of the plant with which we are all more or less 

 familiar, but which are so common that we usually give them 

 very little consideration. Among the most important of the 

 tropisms are: (1) Geotropism, which refers to the downward 

 movement of roots and appears to be due to the force of gravity' 

 as the stimulant. (3) Heliotropism (or phototropism) , which 

 refers to the movements of organs, especially the leaves, towards 

 simlight, which is the stimulant. These two tropisnas result in 

 bringing the stems of most plants into a more or less upright 

 position, although it must be remembered that some stems are 



