CHAPTER XXXVII 

 PLANT MOVEMENTS 



The wide variety of movements which occur in the organs of the higher 

 plants usually escape notice because of the slowness with which they take 

 place. By employing the technique of modern motion picture photography, 

 however, it is possible to demonstrate the movements of plant organs in a 

 spectacular manner. If a growing plant is photographed at regular and 

 frequent intervals for a period of several weeks with a motion picture camera 

 and the resulting film run through a projecting machine all the movements 

 which occurred during several weeks of growth take place within a period 

 of a few minutes. In this way the vigor and reality of the autonomous move- 

 ments of leaves and stems can be demonstrated in a striking manner. The 

 large leaves of tobacco plants, for example, appear to rise and fall almost like 

 the wings of a bird in flight. Likewise the stem tip is seen to participate 

 in more or less regular spiral movements and the several types of movements 

 associated with the expansion of young leaves are vividly portra}ed. The 

 movements that occur during the opening of flower and leaf buds can also 

 be demonstrated by this technique. Anyone who has seen such pictures can 

 not fail to be impressed with the many kinds of movements which occur in the 

 aerial organs of plants and with the magnitude of such movements. 



Classification of Plant Movements. — Most of the movements exhibited 

 by the organs of the higher plants may be classified as : ( i ) growth move- 

 ments, (2) turgor movements, and (3) hydration movements. 



I. Groii'th Movements. — Grov/th movements are changes in the position 

 of organs resulting from an enlargement of cells, or from an increase in the 

 number of cells, or both. Curvatures or other changes in position result 

 from growth movements when the increase in size or number of cells is not 

 uniform at all points in the region undergoing growth. Growth movements 

 are usually divided into three sub-groups: (i) tropic (or tropistic) move- 

 ments, (2) nastic movements, and (3) nutations. 



Tropic movements are those which occur under the influence of environ- 

 mental factors that act with a greater intensity from one direction than from 



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