47] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 207 



plasmic filaments termed cilia (see p. 51) ; each cell may have 

 one, two, four, or many cilia (see Figs. 1, 62, 63). 



Locpmotion is also exhibited by other Algae, such as Diatoms, Oscil- 

 latorias, etc., but the mechanism is not fully understood. 



B. Movements of Cellular Members. Instances of the move- 

 ment of parts of plants consisting of one or more cells having a 

 cell-wall, are afforded by all growing members, and by some 

 specially modified mature members ; the movements of the latter 

 are termed movements of variation, those of the former, movements 

 of growth. 



These two kinds of movements can be readily distinguished 

 from each other, inasmuch as the movements of variation are 

 rapid and can be easily observed, whereas the movements of 

 growth are slow and can only be followed by means of special 

 apparatus. 



a. Movements of Variation. The majority of the movements of 

 variation are induced, a few only being spontaneous. An instance 

 of spontaneous movement is afforded by the rising and falling of 

 the lateral leaflets of the trifoliolate leaf of Desmodium gyrans, the 

 Telegraph-plant. It must, however, be pointed out that the power 

 of spontaneous movement may be possessed by plants though they 

 do not manifest it under ordinary circumstances. Thus the leaves 

 of the Sensitive Plant (Mimosa pudicd) move spontaneously in 

 darkness, but they will not do so in the light. This is also true 

 of various Leguminosce and Oxalidacese. 



b. Movements of Growth. Before entering upon a description 

 of the movements of growth, a clear idea must be formed of what 

 growth really is. By growth is meant change of external form, 

 which is usually, though not necessarily, accompanied by increase 

 in bulk ; the change of form being rendered permanent by the 

 deposition of new substance : it is a function of embryonic proto- 

 plasm (see p. 8). 



The growth of the plant-body takes place to a greater or less 

 extent in all three dimensions of space. For instance, when it 

 takes place equally in all three dimensions, a spherical body is 

 produced, as in Protococcus and Volvox. Occasionally it takes 

 place especially in two dimensions, the result being a flattened 

 body, such as a Fern-prothallus or an Ulva. More commonly, 

 however, it takes place especially in one direction, so that the 

 plant-body assumes an elongated form. An extreme case of this 



