184 



BOTANY 



PART I 



pressure by increasing the external pressure, so that it becomes equal 

 to or greater than the internal. This can be most simply done by 

 bringing the cell into a solution instead of pure water. For simplicity 

 we may assume that the substance in the solution is the same as that 

 within the vacuole of the cell ; in that case, if the concentration of 

 the two solutions is equal, the pressure within and without the cell 

 will be the same, and there "will be no distension of the cell-wall. If, 

 however, the external pressure is increased, the vacuole will diminish 

 in size until the same concentration of the solution inside and outside 

 is attained. The protoplasm follows the diminishing vacuole, but the 

 cell-wall is unable to do so, and there thus results a separation of the 

 protoplasm from the wall. This is known as PLASMOLYSIS (Fig. 177). 



Fir;. 177.— A young cell from the cortical parenchyma of the flower-stalk of Cephalaria leitcantho. 

 m, cell-wall; pi, protoplasm; v, vacuole; /, in water; II, in 4 per cent potassium nitrate 

 solution ; III, in (i per cent solution ; IV, in 10 per cent solution. (After De Vries.) 



It commences at the angles of the cell, but later the protoplasm 

 separates entirely from the cell- wall, and lies free in the cavity as a 

 spherical or ellipsoidal body. 



What particular substance is employed to bring about plasmolysis 

 is on the whole immaterial ; it must not injure the protoplasm, and 

 the latter must be impermeable to it. In many cells a 1-2 ^ 

 solution of potassium nitrate will bring about the commencement of 

 plasmolysis. Cells with higher osmotic pressure require a more 

 concentrated solution. The effect of the solution dejiends entirely on 

 the number of molecules and ions that it contains. For this reason a 

 1 ^ solution of potassium nitrate corresponds to 1'7 y common salt, 

 2*7 y grape-sugar, or 5 'IS y of cane-sugar. 



On transference to pure water the turgcscent condition will be regained, if 

 the protoplasm has not been injured by the solution. If the protoplasm is killed, 

 however, it has become completely permeable, and the necessary condition for a 

 one-sided pressure has disappeared. Fresh, living slices of the Sugar Beet and of 

 the Beet Root when ])laced in pure water allow neither the sugar nor the colouring 

 matter to escape from the uninjured cells. If the protoi)lasm is killed, the sugar 



