226 



BOTANY 



PART I 



the outside or not. A wilted cell which has lost water has more 

 concentrated cell sap, and in the chemist's sense the osmotic pressure 

 has increased. On the other hand, the pressure acting on the mem- 

 brane has diminished or is completely eliminated. This latter pressure, 

 i.e. the externally effective osmotic pressure, may be distinguished as 

 TURGOR PRESSURE, since it brings about the stiffness or turgescence of 

 the plant. A statement that a cell has a certain osmotic pressure 

 thus tells nothing as to the height of the turgor pressure ; this will 

 vary according to the water supply. Given a sufficient supply of 

 water it is true that the whole osmotic pressure will be expressed as 

 turgor pressure. 



The phenomenon known as PLASMOLYSIS serves to determine the 

 osmotic pressure. If a turgescent cell is placed in a salt solution 

 which has a higher osmotic pressure than the cell sap, the pressure on 



FIG. 238. A young cell from the cortical parenchyma of the flower-stalk of Cephalaria leucantha. 

 m, .Cell wall ; pi, protoplasm ; v, vacuole. 7, In water ; II, in 4 per cent potassium nitrate 

 solution ; 777, in 6 per cent solution ; IV, in 10 per cent solution. (After DE VRIES.) 



the membrane is removed and there is a shortening of the cell 

 followed by a separation of the protoplasm from the wall ; this 

 begins at the angles and ultimately leads to the rounding off of the 

 protoplast within the cavity of the cell (Fig. 238). It is immaterial 

 what substance is employed to produce plasmolysis, but the proto- 

 plasm must be impermeable to it and not injured by it. The reason 

 why the solution withdraws water from the cell sap is readily under- 

 stood. Since the external solution contains more molecules and ions 

 than the internal solution, the water in it is less concentrated ; water 

 therefore passes from the higher concentration to the lower until 

 the concentration within and without is the same. If the solution 

 employed for plasmolysis just effects the separation of the protoplasm 

 at the angles of the cell, it can be regarded as isosmotic with the cell 

 sap. Since the osmotic pressure of the solution is known from 

 physical investigations, we thus arrive at the osmotic pressure in the 

 cell. Plasmolytic determinations have shown that in ordinary cells 



