154 PHYSIOLOGY OF NUTRITION. 



61. The Magnitude of the Osmotic Pressure. 



To determine the magnitude of the active osmotic pressure in 

 plant structures we may experiment with shoots, employing pieces 

 1 or 2 mm. in average diameter and 100 mm. long; e.g. flower 

 scapes of Plantago, or stems of Lonicera tatarica (which I used), 

 etc., etc. On these we paint ink-marks, at distances of 80 mm. 

 from each other, and then thoroughly plasmolyse them by twenty- 

 four hours' exposure to the action of 10 per cent, solution of 

 common salt. The shortening which is brought about can easily 

 be determined by means of a millimetre scale. The shoots are 

 now stretched by means of the apparatus drawn in Fig. 51. They 

 rest horizontally on a board, 13, or, better still, on a small sheet 

 of cork. Their thin end is covered with a small block of cork, 

 Jf, which we can firmly fix in position by means of a screw, 

 and round their thicker end is tied a 

 piece of string. The string runs over a 

 pulley, .R, and supports a scale-pan, (7, 

 for weights. We load the scale-pan until 

 the distance between the marks on the 

 shoots has become the same as before 

 plasmolysis, viz. 80 mm. That which 

 in the experiment is effected by the 

 weights, is effected in nature by the 

 osmotic pressure. We can thus ex- 

 perimentally determine the magnitude of 

 the pressure within the uninjured plant 

 FIG. 51. Apparatus for de- structure with at least approximate 



terminin* the amount of os- accm > jf the mean diameter of a 



motic pressure. ? 



cylindrical piece of stem is 1 mm., 



the corresponding area of its transverse section is 0*785 sq. mm., 

 the area of a circle being given by the formula ?rr 2 (7r = 3'141). 

 If 50 grams are required to stretch to its original length a plas- 

 molysed shoot 1 mm. in diameter, the osmotic pressure within 

 the fresh shoot is equal to about 6| atmospheres, and, in fact, it 

 frequently attains as high a value as this. In one particular 

 instance, in a stem of Lonicera tatarica, I found it to be 1*4 

 atmospheres. 1 



1 See H. de Vries, Die mechanischen Ursachen der Zellstreckuny, Halle, 1877, 

 p. 118. 



