78 PLANT PHYSIOLOGY 



(c) Osmosis may be demonstrated by tying a piece of fresh 

 bladder securely across the mouth of a thistle tube which is 

 inverted and filled with a strong solution of sugar up to a mark 

 on the stem. The larger end with the bladder is now placed 

 in a dish of water so that the water outside stands at the same 

 height as the water inside. The water will enter through the 

 bladder by osmosis and ascend the stem, perhaps reaching a 

 height of a meter or more above the level of the water outside. 

 The more impermeable the membrane is to the substance in 

 solution while still remaining permeable to water, the greater 

 the difference in level and the higher the pressure 

 that can be obtained. The latter can be measured 

 roughly by connecting the stem of the thistle tube to 

 a mercury manometer. 



(d) The relation of osmosis to turgor may be demon- 

 strated by making an "artificial cell." Fill a test 

 tube with a strong sugar solution and tie a piece of 

 bladder firmly over the open end. Place in a dish of 

 water. The water that passes into the tube by osmo- 

 sis through the bladder causes the latter to be 

 stretched and to bulge out. On removing the tube from the 

 water, and pricking the bladder with a pin, the pressure 

 developed by the stretching of the bladder will force the water 

 out in a stream. 



(e) Mount one or two filaments of Spirogyra in water and 

 examine. Measure the length of a portion including a definite 

 number of cells. Now draw a 2 per cent, potassium nitrate 

 solution or a 5 per cent, sugar solution under the cover glass by 

 adding it at one side and withdrawing the water from the 

 other side with a piece of filter paper. Measure the filament 

 again. Add increasingly strong solutions and when the right 

 strength is reached, the cytoplasm will be found to be drawing 

 away from the corners of the cell wall, i.e. plasmolysis has 

 begun. This indicates that with the withdrawal of water by 

 the solution outside, the much stretched cell walls have lost 

 their tension until they have reached a state in which they are 

 not at all stretched. As the water is still withdrawn from the 

 cell, the cytoplasm is pulled further and further away from the 

 wall. At this stage, again measure the filament and calculate 

 the amount that the turgid filament was stretched. 



(/) To demonstrate that evaporation from a membrane filled 



