THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 277 



IV. THE PHYSIOLOGY OF PLANT-MOVEMENTS. 



A. CLASSIFICATION OF MOVEMENTS ACCORDING TO CAUSE. THE 

 OUTWARD MANIFESTATION OF SOME MOVEMENTS. 



The consideration of the outward manifestation of movements 

 in plants does not aid us in forming a rational classification of 

 the same. The following will explain. 



In general, nutation implies the curvature of an organ. To 

 bring this about it is necessary that one side should always be 

 relatively longer or shorter than the opposite side. Elongation 

 of tissues (cells) may be due to growth or to the taking up of 

 water without growth. Shortening is usually due to changes in 

 the amount of water present, but may also be due to other causes. 

 If the line of maximum expansion changes from side to side 

 (with or without growth), it will cause the organ to move back 

 and forth like a pendulum. The leaflets of Hedysarum gyrans 

 describe elliptical curves (without growth). 



When the longitudinal axis of maximum growth or maximum 

 expansion remains neither on one side nor alternates from one 

 side to the other, but rotates or ivinds about in the organ, it pro- 

 duces what is known as ctrc^mnutation. The organ is thereby 

 carried around in a circle ; the growing tip describes a spiral 

 line. 



Torsions also belong to the important phenomena of move- 

 ment. They may also be the result of a variety of causes. The 

 locomotor movements are characteristic because of their external 

 peculiarity : entire plants or parts of plants may move about. 

 The movements of entire plants may be explained mechanically 

 when cilia can be demonstrated, as in various swarm-spores and 

 bacteria ; if cilia are wanting, as in Diatomacece, Myxomycetes, 

 etc., it is difficult or impossible to explain the motion. As 

 motion of parts of plants the movement of chlorophyll and plasmic 

 motion may be mentioned as an example. The former we can 

 comprehend at least from a teleological point of view. (See Func- 

 tion VI in the Physiology of Tissues, Part II, B.) The causa 

 efficiens as well as the causa finalis of plasmic movement is un- 



