I20 OUTLINES OF PLANT LIFE. 



in active motion, or when they extend a large portion of 

 the body for creeping (fig. no). The extension of such 

 organs, whether slender or thick, is directly opposed by 

 strong physical forces acting at the surface which tend to 

 contract the body into a sphere, as they do a drop of liquid. 



Fig. iio. — Plasmodia, creeping bits of nalced protoplasm, sliowing varied shapes as 

 parts are protruded or witlidrawn. Highly magnified,— .4£ter Kerner. 



156. 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 protoplasm. 

 In the interior of the protoplasm there exist one or more 

 water chambers, the vacuoles (^ 4, and fig. 117). 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 diminished and the protoplasm shrunken away 

 from the wall; remeasurement will show that the cell has 

 diminished both in length and diameter. In its normal con- 

 dition, therefore, the wall was stretched by the pressure of 

 the contents within. If a cell which has been thus shrunken 

 by immersion in a solution of salt be again placed in water, it 

 may regain, in the course of a few hours, its original condi- 

 tion, that is, it may again become turgid. This would be 

 brought about by the entrance of water into the vacuoles to 



