14 Practical Plant Biology. 



dissolved particles there are in unit volume. Measurement has also 

 shown that the osmotic pressure is directly influenced by temper- 

 ature, so that the osmotic pressure of dissolved particles is very 

 similar to the pressure produced by gas particles, and obeys 

 similar laws. At ordinary temperatures the osmotic pressure of a 

 5 per cent, solution of cane sugar is about 3 atmospheres, and 

 that of a 5 per cent, solution of common salt is about 40 atmos- 

 pheres. 



Membranes which, like those used in these experiments, obstruct 

 the passage of diffusing solutes while they are permeable to the 

 solvent, water, are called semi-permeable. In many cases it is 

 found that they do not absolutely preclude the passage of the 

 solute, and it is found that some sugar passes out through the 

 membrane during the observations. But other membranes are 

 known which completely obstruct the passage of certain solutes 

 and every degree of semi-permeability may be met with. 



Protoplasm normally exhibits a very perfect semi-permeability. 

 Hence the dissolved substances in the vacuoles of cells cannot es- 

 cape from the cells when they are surrounded with water but tend 

 to push the protoplasm out against the cell-wall. The protoplasm 

 being semi-liquid, would flow before the pressure but is held up to 

 it by the cell-wall and the osmotic pressure of the solutes of the 

 vacuoles is then balanced by the tension of the cell-wall. 



But we may also balance it by an external osmotic pressure. 

 This may be done by immersing the tissue in a solution having 

 an equal or greater concentration than that in the vacuole. A 

 solution of a non-poisonous substance should be used. If a thin 

 slice of beetroot, or thin flap torn off a rhododendron petal is 

 mounted in a drop of a 10 per cent, solution of common salt, you 

 will see, as you observe it in the microscope, that the rose-coloured 

 vacuoles, which at first entirely filled the cavities of the cells- 

 save for the space occupied by the delicate containing film of 

 protoplasm are now contracting into small densely coloured 

 globules in the centre of the cell cavity. The explanation of this 

 contraction is as follows : The external salt solution has diffused 

 through the permeable cell-walls and come into contact with the 

 film of protoplasm. The diffusing particles not being able to 

 penetrate the protoplasm push against it and, being more con- 

 centrated than the dissolved particles in the vacuole, exercise a 

 superior pressure and drive the film away from the wall into the 

 cavity of the cell. A cell in this state is said to be plasmolysed. 

 It is evident that while the external osmotic pressure is thus 

 driving in the protoplasm it reduces the size of the vacuole, and 



