VARIOUS TYPES OF MOVEMENT 543 



application for the external agencies that actually effect the transporta- 

 tion ; this type of adaptation is exemplified by the various contrivances 

 which facilitate the dispersal of seeds or fruits by wind. Ejection of 

 seeds or spores is often also brought about by the sudden release of 

 tension previously set up by the action of turgor, or in consequence 

 of the cohesive power of evaporating cell-sap. Many reproductive 

 organs are furnished with hygroscopic arrangements, which assist in 

 the distribution of the spores or seeds. 



The various parts of the plant-body perform a great number of 

 movements of different kinds, which are in no way concerned with 

 locomotion or with the transportation of seeds or spores, but which 

 serve to effect a suitable adjustment in space of the individual organs. 

 Movements of this type are constantly performed by every highly 

 organised plant. Some of them, such as the curling and uncurling of 

 the leaves in certain xerophilous Grasses, are due to purely physical 

 causes. As a rule, however, such adjustments are special manifesta- 

 tions of vital activity, and the energy required for their performance is 

 derived from the metabolic activity of living protoplasts. External 

 agencies act in general merely as stimuli, which may, however, not 

 only induce an organ to move, but also determine the direction in 

 which the movement takes place. 



In a number of cases such active movements are carried out 

 without the aid of special motor-tissues or -organs. The curvatures 

 associated with heliotropic and geotropic response, for example, 

 are generally performed by organs which are still engaged in longi- 

 tudinal extension, and by tissues which will be concerned with 

 totally different functions when they are fully developed. Frequently, 

 however more especially in connection with foliage leaves special 

 motor-organs are differentiated ; these are usually capable of executing 

 repeated movements. Such motor-organs, which may occur on stems, 

 petioles, leaf-sheaths, or pinnae, are termed pulvini. In addition to 

 the actual motor-elements, they naturally contain other tissues 

 belonging to the dermal, skeletal, conducting and other systems, for 

 the same reason that a photosynthetic organ is not exclusively 

 composed of photosynthetic cells. 



The motor system includes all tissues and anatomical features 

 which are primarily concerned with the execution of passive or active 

 movements. It will be found convenient to distinguish between active 

 and passive motor- tissues. In the case of the latter, the movement is 

 carried out with the help of external forces, whereas it is characteristic 

 of the active type that the requisite energy is provided by the tissue 

 itself. The flying-hairs and -tissues which assist in the dispersal of 

 seeds and fruits by wind, thus belong to the class of passive 



