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) Therelation of osmosis to turgor may be demon- 
<i| strated by making an “artificial cell.” Fill a ‘test 
Fie. 40, tube with a strong sugar solution and tie a piece of 
Osmosis bladder firmly over the open end. Place in a dish of 
ment (c), 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 As the water is still withdrawn from the 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. 
(f) To demonstrate that evaporation from a membrane filled 
