148 



PLANT LIFE. 



in active motion (fig. 168), or when they extend a portion of 

 the body as a pseudopodium (fig. 169). 



FIG. 169. Plasmodia, creeping bits of naked protoplasm, showing varied shapes as 

 parts are protruded or withdrawn. Highly magnified. After Kerner. 



188. Turgor. If the organism be one surrounded by a 

 cell-wall, or if it be made up of a number of cells united, the 

 cell-wall itself plays a considerable part in maintaining the 

 form. This is due to the condition of the cell known as 

 turgor. When fully mature the cell-wall of each active 

 cell is lined by a more or less thick layer of living proto- 

 plasm. In the interior of the protoplasm there exist one or 

 more water chambers, the vacuoles (^[5). If such a cell as 

 this be measured in its normal condition, and then surrounded 

 for a few moments by a 10 per cent, solution of common salt, 

 re-examination will show that the vacuoles have been dimin- 

 ished, the protoplasm shrunken away from the wall, and 

 remeasurement will show that the cell has diminished both in 

 length and diameter. In its normal condition, therefore, the 

 wall was stretched by the pressure of the contents within. If 

 a cell which has been thus shrunken by immersion in a solu- 

 tion of salt be again placed in water, it may regain, in the 

 course of a few hours, its original condition, that is, it may 

 again become turgid. This would be brought about by the 

 entrance of water into the vacuoles to replace that withdrawn 

 when the cell was placed in the solution of salt. 



If a thin piece of rubber tubing be connected with a pump 

 and filled with water until it is stretched, it increases its 



