WIBTING AND PLASMOLYSIS lOQ 



and herbaceous plants depend upon it for maintain- 

 ing their stiffness and the erect position of their 

 shoots. 



Wilting and Plasmolysis. — If a plant cell loses water 

 faster than it can obtain it, turgidity cannot be main- 

 tained, the cell becomes soft and limp, and the tissue 

 loses its rigidity. This is what happens on a very 

 hot day when plants lose water by evaporation to the 

 air faster than they can replenish their supply by root 

 absorption. The plant is said to wili. The loss of 

 turgidity takes place even more rapidly when a plant 

 is pulled up by the roots and left in the sun. 



Turgidity can also be lost in another way. If the 

 cell or tissue be placed in a sugar or salt solution of 

 osmotic strength greater than that of the cell sap, 

 water is drawn out of the cell against the pull of the 

 cell sap. The protoplasm is then no longer held tight 

 against the wall by the pressure from within, and it 

 contracts away from the wall as the liquid pressure 

 within diminishes by the withdrawal of water. This 

 process is called plasmolysis.^ The osmotic substances 

 of the cell sap do not, however, pass out through the 

 semi-permeable membranes represented by the vacuole 

 wall and ectoplasm, and if the cell or tissue be plaCed 

 again in water this re-enters the cell, whose turgidity 

 is re-established. 



PRACTICAL WORK. 



(i) Examine the growth of the bean roots set up in the corked 

 bottle last time. They show {a) local growth, (b) curvature 

 from the horizontal to the vertical plane, i.e. the bending of an 

 organ in response to an external force. Sketch diagrammatically 

 these results alongside the sketches of the roots as originally 

 set up. 



' Greek AiSfftj, loosening of the plasma from the wall. 



