OSMOTIC PRESSURE: SUCTION FORCE 51 



Hannig (19 12) measured the osmotic pressures of the roots 

 of a large number of plants ; he used the cortical cells of the 

 roots, as the root hairs of plants taken from the soil are too 

 difficult to work with. But he remarks that occasional 

 observations on the root hairs showed the osmotic pressure 

 to differ little from that of the cortical cells. Table IX gives 

 his results for a number of plants growing in different 

 conditions in the Strasburg Botanic Gardens. 



TABLE IX 

 Osmotic Pressures of Root Cells 



For salt-marsh plants measurements have been made 

 by T. G. Hill (1908), who found the osmotic pressure of 

 the root hairs of Salicornia and of Suaeda equivalent to 

 67 per cent, sodium chloride; a mesophyte (see pp. 199,200) 

 seedhng from adjacent ploughed fields gave a value of 1*5 

 per cent, sodium chloride. These amounts indicate osmotic 

 pressures of 41 and 9'! atmospheres respectively. The 

 values for the salt-marsh plants are thus very high, and this 

 is of course related to the fact that they draw their supply 

 from salt water with a high osmotic pressure. The water 

 of the marshes may have a salt content of 3 '67 per cent. 

 The root hairs of these plants have great powers of accom- 

 modation. After soaking in fresh water for twelve hours, 

 the osmotic pressure may fall as low as 11 "3 atmospheres. 

 This power of accommodation may be important in nature, 

 as the salt content of the marsh water is subject to great 



