Minnesota Plant Life. 501 



Ijlades keep up a flapping motion, which, aUhough slow, is 

 plainl)^ visible. Of what use this may be, no one clearly knows. 

 Possibly it serves to frighten away leaf-cutting ants, but this 

 seems doubtful. In any event such a movement is typically 

 automatic. It takes place only under suitable conditions of 

 temperature and nutrition, but these conditions cannot be 

 termed the cause of the movement. That, indeed, must be 

 very complex and should be sought in the intimate structure 

 of the plant. 



Induced movements of organs are numerous — so numerous, 

 indeed, that it may be said of every organ that it is capable 

 of making a variety of irritable responses to the impulses reach- 

 ing it from the outer w'orld. Among the various effective 

 stimuli are the forces of gravity, light, heat, magnetism and 

 electricity, also air, moisture, matter in solution, matter in 

 contact, and various mechanical tensions, pressures and strains. 



While not the only methods of response, very characteristic 

 reactions of organs to an outward stimulus are by maintenance 

 of positions parallel with, or perpendicular to, the force-lines 

 of the stimulus, or by curvature in some definite direction. 

 Thus, under the constant action of the force of gravity, shoots 

 ordinarily maintain an erect position, growing against the force ; 

 while roots grow zvitJi the force. Both positions are manifesta- 

 tions of irritability. In the various plant-positions, there is 

 also to be distinguished an automatic type of irritability. Thus, 

 quite apart from the force of gravity, root and shoot grow in 

 opposite directions. They will do this if whirled on a hori- 

 zontal wheel, as in the experiment of Knight, who thus demon- 

 strated the similarity between the response to centrifugal force 

 and that to gravity, for in such an apparatus shoots grow 

 towards the center and roots towards the rim. There is, so to 

 speak, a polarity in plant organs. They have tips and bases, or 

 upper and under sides, or fixed and free ends. In the main- 

 tenance of polarity automatic influences are at work. For this 

 reason counteraction of the force of gravity, by revolution of 

 the plant upon a vertical wdieel, does not permit the plant or- 

 gans to become shapeless and unrecognizable. On the con- 

 trary, they retain their polarity, though their direction may 

 be different. 



