128 PLANT PHYSIOLOGY 



which were in a definite ratio to each other. As the years have 

 passed the total quantities have increased but the ratios have re- 

 mained fairly constant. Similarly the ratios of the various salts 

 in the blood to each other are approximately the same as those 

 of the salts in the sea; the concentration in the blood is much less 

 than that in the ocean, and probably represents the concentration 

 in the sea in the far-away Cambrian period when animal life first 

 developed on the land. 



These salts in plants fulfill three different functions. (1) Some 

 are used in the chemical composition of the plant body and its 

 products. To that extent they are nutritive. (2) Some are probably 

 catalytic in their action and, while they do not enter into the final 

 products, they cause these reactions to come to pass. (3) Other 

 salts are purely balancing in their action. They antagonize the 

 harmful effects produced by others and furnish a medium of the 

 proper acidity and hydrogen ion concentration for the chemi- 

 cal reactions necessary for optimum growth and development. 

 In its nutritive effect a salt cannot be replaced by any other one. 

 In its catalytic effect there is evidence that it may be so replaced, 

 while in its balancing effect there is no doubt but that it can be 

 replaced by a number of other salts. This explains why potassium 

 can be replaced partly by sodium but not entirely. Sodium can 

 take the place of potassium as an antagonizer of the harmful 

 effects of calcium but it cannot replace potassium in its beneficial, 

 nutrient, chemical reactions. It may also develop that the toxicity 

 of magnesium chloride is due partly to its electrical charge and 

 partly to some other effect; the former is antagonized by the 

 sodium and the potassium chlorides while the latter is antagonized 

 by the calcium chloride. 



As mentioned in the preceding chapter it is often difficult to 

 distinguish a balancing from a nutrient effect. Thus it has been 

 noted that if fruit trees are lacking in potassium, brown patches 

 appear upon the leaves, which are followed by premature leaf 

 fall. This seems to be a nutrient effect, but if there is too much 

 nitrogen in the supply of nutrients, the addition of potassium does 

 not help. There is thus seen to be some more complex difficulty 

 than the mere absence of potassium as a nutrient. 



Theoretically, there should be some best concentration of these 

 various elements for each plant, but practically this has been found 

 very difficult to ascertain, owing to the effect of the various factors 



