MOVEMENT 359 



they are found along the walls which are parallel to the rays of 

 light, thus protecting themselves as much as possible from its 

 harmful action (Fig. 26) . 



External Movements; Change of Position. — The externally 

 visible movements of plants are those commonly included in a 

 discussion of motion. They result, either in an actual change of 

 position, if the organism is not fixed to the earth, or to curvatures, 

 in case the plant is attached in such a way that the entire body 

 is not free to move. This movement of the entire organism may 

 be due to the peculiar methods of growth of the plant or to inde- 

 pendent locomotion similar to that of animals. Those organisms 

 which move through growth generally possess underground stems 

 which run parallel to the earth, sending down roots into the soil 

 and sending up aerial shoots to procure access to air and light. 

 The stems frequently die off behind as growth continues, with 

 the result that the plant slowly marches across the terrain as the 

 forward end grows and the posterior end disintegrates. A fern 

 may move in this fashion five cm. a year, which makes a meter 

 in twenty years. Although this rate is rather slow, time is not of 

 paramount importance to a fern. Iris, Nymphdea, and others may 

 move in this same manner. 



Independent Locomotion. — The independent movements in 

 plants are only to be found among the simpler forms. As stated 

 in Chapter VII, animals and plants have evolved along two differ- 

 ent lines,— the animals towards motility and the plants towards 

 stability,— these differences being associated with the manner of 



procuring food. 



Three types of movements are to be found,— amoeboid move- 

 ments, ciliary movements, and excretory movements. The first 

 derive their name from the Amoeba, an animal which exhibits 

 better than plants this motion characteristic of the simplest 

 organisms known. The chief plants to show this type of move- 

 ment are the plasmodia of the Myxomycetes which, because of 

 their many animal characters, are sometimes included in the 

 animal kingdom. These organisms are naked masses of proto- 

 plasm which live in wet places on decaying wood, wet leaves, etc. 

 When locomotion occurs, the lobes of the protoplast are advanced 

 along one side forming a protrusion into which the rest of the 

 protoplasm slowly streams. In this way the entire mass advances 

 in the direction of these protruding lobes or pseudopods. 



