184 



PROTOPLASM 



tube, tightly fastened, projects. A concentrated (20 per cent) 

 solution of sugar or pure molasses is put in the thistle tube or 

 sack, which is then immersed in water. Such an apparatus 

 is termed an osmometer, or osmoscope. (The latter term was 

 suggested by Ganong because, strictly, an osmometer must be 

 capable of measuring (maximum) osmotic pressure, which the 

 usual osmotic sack does not do.) Water enters the thistle or 

 sack and rises in the tube, climbing higher and higher, and thus 



by its own weight creates a 

 pressure — a hydrostatic pressure. 

 If the tube or sack is tightly 

 corked at the top and full at the 

 beginning of the experiment, 

 some water will still enter and 

 create a pressure within. There 

 is no pressure until water has 

 entered. This pressure causes 

 turgor. 



The living plant cell (Fig. 4) 

 is just such an osmoscope, made, 

 to be sure, of other material but 

 functioning in the very same 

 way and developing a pressure 

 within the cell. The sap in the 

 cell vacuole corresponds to the 

 sugar solution; the protoplasmic 

 membrane around the vacuole 

 (or the outer membrane around 

 the protoplast) is the semiper- 

 meable membrane. It is kept 

 from bursting by the cell wall of 

 cellulose. The surrounding water 

 within the plant body, or in the 

 soil or pond, is the outside' 

 aqueous medium. The pressure developed by plant cells may 

 be very high, usually about 10 atmospheres. 



A classical and picturesque demonstration of an osmotic sys- 

 tem is that in which the selective membrane is one of copper 

 ferrocyanide. (It was first made by M. Traube, in 1867.) 

 Crystals of copper sulphate (CUSO4) are dropped in a solution of 



Fig. 100. — An osmotic system 

 consisting of a thistle tube with 

 mouth covered by a membrane; 

 sugar solution within and water 

 without. 



