CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT, 737 



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. Instances 



I of spontaneous movement are afforded by the rising and falling of 

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

 Telegraph-plan^f hy the oscillation of the labellum of Megaclinium 

 falcatum, an African Orchid; by the bending upwards and down- 

 wards of the gynostemium of Stylidiicm (CandoUea) adnatum 

 (see p. 692). It must, however, be pointed out that the power of 

 spontaneous movement may be possessed by plants tliough they do 

 not manifest it under ordinary circumstances. Thus the leaves 

 of the Sensitive Plant (Mimosa pudica) move spontaneously in 

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

 of various Leguminosae and Oxalidaceae. 



b. Movements of Groivth. 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. 16). 



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 

 is afforded by Spirogyra and other filamentous Algce. It is this 

 growth in length which has been more especially studied physio- 

 logically, and in what follows, " growth " may be taken to mean 

 '* growth in length," unless there is some definite statement to the 

 conti^ry. 



