192 



PLANT LIFE. 



within the cell, such as the nucleus and the chloroplasts, are 

 cajjable of moving about. Under moderate illumination 

 chloroplasts accumulate upon the sides of the cells most di- 

 rectly reached by the light. Under very strong illumination 

 they retreat to the walls least illuminated, or may even pile 

 up in the angles of the cell so as to shade each other (fig. 

 189). 



Fig. 189. — Cells from tlie spongy parenchyma of the leaf fo wood sorrel {O.ralis), seen 

 from the direction in which light falls on the leaf. ((, position of the chloroplasts in 

 diffuse light ; /', position after shoil exposure to direct sunlight ; l , position after longer 

 exposure. Highly magnified. — After Stahl. 



III. Movements of multicellular members. 



281. Forces. — The moveiuents of multi(.ellular parts may 

 be brought about either by special organs known as motor 

 organs, or by the growth of the immature parts. Motor or- 

 gans are generally resjionsible fVir the movements of mature 

 parts, while irio\emcnts of the younger regions are generally 

 due to growth. The force e.verted by the motor organs is 

 dependent upon the altered turgor of the cells of which the 

 organ is composed. If the cells upon one side lose their tur- 

 gidity, those upon the othci-, being unresisted, will extend 

 and bend the organ toward the side upon which the turgor 

 was diminished. It will be con\'enient, therefore, to dis- 

 tinguish mo\'ements due to growth and mo\'ements due to 

 variation in turgor. 



282. ( A ) Movements of growth. — These depend upon 

 soiin; iiu'i|uality in tlie rate of growth of the organ concerned. 

 Thf\' are of two sorts: (i) those in which ^-ariation ingrowth 



