i8o OSMOSIS : ABSORPTION OF WATER 



cell-wall, but none is allowed to pass through the living cytoplasm. 

 Moreover, the osmotic substances dissolved in the cell-sap do 

 not travel outwards through the cytoplasm. Cells plasmolysed 

 in this manner regain their turgid condition when placed in pure 

 water ; the plasm olysing substance which has passed through the 

 cell-wall diffuses out and water again enters the vacuole so that 

 the cytoplasm becomes forced into contact with the cell-wall. 



When a leaf or a branch with leaves upon it is cut from a 

 plant and left exposed to the air, water soon escapes from the 

 cells in the form of vapour ; the turgidity of the cells is rapidly 

 reduced and, in consequence, the leaves instead of maintaining 

 their elasticity and firmness, become limp and unable to support 

 themselves in a normal position. 



This flaccid state of ' wilted ' or ' faded ' parts of plants is always 

 brought about by the loss of water from the cells whereby their 

 turgid stretched condition is reduced, although the conditions 

 which lead to the loss of water is not the same in all cases. 



If the loss of water from a cut shoot has not gone too far, and 

 the cytoplasm is still living, it is generally possible to renew the 

 former turgid state of its cells by placing the end of the stem in 

 water, or by forcing water into the 'wilted' shoot as in Ex. 105. 



From various extensive observations and experiments it is 

 evident that the passage of substances in solution into or out 

 of a cell, is under the control of the living cytoplasm; the 

 phenomena of turgidity and other osmotic properties are de- 

 stroyed when death of the cytoplasm takes place. 



Ex. 99.— Stretch a piece of wetted bladder across one end of a glass lamp- 

 chimney and firmly tie it with string ; then fill about J of the chimney with 

 a saturated solution of sugar, and suspend it in a vessel of water, so that 

 the sugar-solution inside the glass chimney is level with the surface of the 

 water outside. Allow it to remain for a few hours ; note that the water 

 passes inwards through the bladder into the sugar solution and causes the 

 level of the latter to rise. 



Ex. 100.— Repeat the preceding experiment, using a strong solution of 

 copper sulphate or potassium bichromate. Observe if the copper sulphate 

 or potassium bichromate passes outwards and colours the clear water. 



