314 TEXTBOOK OF PLANT PHYSIOLOGY 



Besides growth movements, which are typical of plants, one 

 may also observe another type of movement resulting from changes 

 in turgor pressure within the cells. These are more rapid than 

 growth movements. Their best-known example is the movement 

 of Mimosa leaves. Not being subject to growth, they may be 

 repeated, and hence are called alternating movements. Only a 

 small number of plants exhibit this phenomenon, while growth 

 movements may be observed in all plants. Still their study has 

 been of very great interest, as in their nature they are nearest to 

 the movements of animals and are conditioned by a comparatively 

 rapid transmission of irritation. 



Some lower plants, particularly those having only one cell, 

 exhibit free locomotion of the whole body by means of special 

 organs of movement, such as the cilia or flagella. These move- 

 ments in no way differ from those of lower animals and hence they 

 will not be discussed here. 



Likewise, no consideration will be given to such changes in 

 position of plant organs as are not caused by physiological, but 

 by purely physical processes, as, for instance, the opening of many 

 fruits during drying (geranium, vetch, Oxalis) the curling of the 

 awns (beards) of wild oats and Stipa; the bending of branches of 

 trees in winter, due to changes in temperature; the splitting of 

 certain juicy fruits (Impatiens), etc. Though many of these 

 movements play an important part in the life of a plant and are 

 frequently connected with a rather intricate mechanism, still it is 

 impossible to regard them as manifestations of the life activity of a 

 plant. 



94. Phototropism. — It has been noted that light is the most 

 indispensable factor for the existence of green plants. It provides 

 them with energy required for the decomposition of carbon dioxide. 

 It is but natural, therefore, that plants should show a high degree 

 of sensitivity to light and should respond effectively to all changes 

 of the direction of light rays falling upon them. 



The capacity of turning toward or from the source of light is 

 known as "phototropism." It was formerly called " heliotropism " 

 (from the Greek word helios, the sun), as daylight was then chiefly 

 used for experimental purposes. Young growing stems usually 

 bend in the direction of light, this reaction being known by the 

 term "positive phototropism" (Fig. 124). The curving of plants 

 away from the source of light is called "negative phototropism," 



