THE MAINTENANCE OF BODILY FORM. 121 



replace that withdrawn when the cell was placed in the solu- 

 tion 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 

 diameter and length slightly, and gains, at the same time, a 

 condition of rigidity greater than in its unstretched condition. 

 In a similar way turgid cells are more rigid than those which 

 are flaccid. The union of turgid cells produces a member 

 more rigid than one in which the cells are not turgid. An 

 illustration of this is to be seen in the condition of a wilted, 

 as compared with a fresh, leaf. The turgor of thin-walled 

 cells may play an important part in maintaining the form 

 and position of the parts of a plant. 



EXERCISE XX. 



Demonstration. To show the existence of turgor in the individual cell. 



Mount a, bit of Spirogyra under microscope ; observe position of 

 chlorophyll bands. Irrigate with 5 per cent, solution of salt and note 

 effect. 



(If Spirogyra is not at hand use hairs on stamens of Tradescantia ; or 

 the epidermis, filled with purple cell sap, from the under side of the 

 leaves of the variegated Tradescantia (" wandering Jew"); or the hairs 

 of geranium leaves. ) 



To show effect of turgor of cells onrigidity of young parts containing no 

 mechanical tissues. 



Remove carefully a young plant with vigorous primary root grown in 

 sawdust or moss. Lay in water for a few minutes. Note rigidity. 

 Transfer to 5 per cent, salt solution for a few minutes. Has rigidity in- 

 creased or diminished ? Remove to water again for 15 min. What is 

 the result ? 



157. Tissue tensions. — But turgor can affect only those 

 cells whose walls are thin and extensible. Those whose 

 walls have become thick and""'rigid are not stretched by this 

 force. In the ..larger plants, however, where both thick- 

 walled and thin-walled tissues exist, it is possible that a mass 



