ACTIVE: pivANT FOOD, ETC. 199 



Plants have the capacity of taking up one or the other of these 

 ions alone. From a solution of potassium chloride, the plant may 

 take up more potassium ions than chlorine ions, leaving hydrogen 

 in place of the potassium. The liquid would then become acid 

 from the presence of hydrochloric acid. The plant may remove 

 the nitrate ions from a solution of calcium nitrate. The calcium 

 ion would then unite with carbon dioxide and form insoluble cal- 

 cium carbonate. 



The above considerations are essentially modified in the pres- 

 ence of two or more salts ; as a rule, when two salts are present, 

 which are required by the plant, their absorption is accelerated. 

 For example, potassium salts are taken up in much larger quantity 

 when a calcium salt is present. Potassium nitrate may be entirely 

 removed from a solution containing calcium nitrate. In the 

 presence of other ions, the potassium ion, nitrate ion, phosphate 

 ion, and sulphate ion can be completely removed from solution, 

 while calcium or magnesium ions become more concentrated in 

 the solution, as a rule. 



A study of the effect of the amount of water evaporated upon 

 the ash constituents taken up has shown that the stronger the 

 evaporation, the more dilute is the solution taken up by the plant, 

 but at the same time the more substance is taken up, since the 

 decrease in concentration is in a less ratio than the increase in 

 water evaporated. 



Soil Solution. The soil solutions are exceedingly dilute. From 

 what has been said, however, it is seen that plants may withdraw 

 nourishment from very dilute solutions. It is self-evident, that 

 when the dilution goes below certain limits, diffusion will not 

 take place with sufficient rapidity to satisfy the requirements of 

 the plants, and limits are conceivable at which only a minimum 

 growth w^ill take place. 



In other words, there is an optimum of concentration, above 

 and below which a lesser production of plant substance takes 

 place. Below, because transpiration and diffusion do not provide 

 sufficient food. 



