ANTAGONISM 15 



antagonism, and is probably greatest between ions of different 

 valencies. A mixture of ions in solution in such proportions 

 that they exert no toxic effect is called a balanced solution, and 

 when a solution of this nature produces contraction of the proto- 

 plast, the effect is a true plasmolysis. In this category may be 

 placed sea-water, which is a mixture consisting mainly of sodium 

 chloride, magnesium chloride, magnesium sulphate, potassium 

 chloride, and calcium chloride, and for this reason sea-water 

 suitably diluted forms the best medium for determining osmotic 

 pressures in the plant-cell. 



The fact that many mineral salts produce false plasmolysis 

 proves that they are capable of penetrating the protoplast ; and 

 even when in so dilute a solution as to have no deleterious effects, 

 there is no reason to suppose that their power of penetration is 

 diminished. The compounds in the soil-water are similarly 

 capable of passing into the plant, although so many are present 

 that the influence of the one upon the other cannot be over- 

 looked. As yet, however, our knowledge on this point is quite 

 inadequate. The complexity of the phenomena is illustrated 

 by the fact that plants do not absorb compounds in the same 

 proportions as they occur naturally in the soil-solution. One 

 of the most striking instances of such selective absorption is 

 furnished by Seaweeds, which contain as much as 0*5 per cent, 

 of iodine in their ash, whilst iodides are present in sea-water in 

 almost imperceptible amount. 



