CHAPTER I 



INTRODUCTION 



EvEE since the physiological side of botany began to emerge from 

 obscurity, the question of the relation between the nutrition and the 

 growth of the plant has occupied a foremost position. All kinds of 

 theories, both probable and improbable, have been held as to the way 

 in which plants obtain the various components of their foods. But quite 

 early in the history of the subject it was acknowledged that the soil was 

 the source of the mineral constituents of the plant food, and that the 

 roots were the organs by which they were received into the plant. 



A new chapter in the?- history of science was begun when Ldebig in 

 1840 first discussed the importance of inorganic or mineral substances 

 in plant nutrition. This discussion led to a vast amount of work 

 dealing with the problem of nutrition from many points^ of view, and 

 the general result has been the sorting out of the elements into three 

 groups, nutritive, indifferent, and toxic. Thus calcium, phosphorus, 

 nitrogen and potassium are classed as nutritive, arsenic, copper and 

 boron as toxic, and many others are regarded as indifferent. 



Closer examination, however, shows that this division into three 

 classes is too rigid. Now that experiments are more refined it has 

 become evident that no such simple grouping is possible. It has been 

 found that typical nutrient salts are toxic when they are applied singly 

 to the plant in certain concentrations, the toxic power decreasing and 

 the nutritive function coming into play more fully on- the addition 

 of other nutrient salts. For instance, Burlingham found that the 

 typical nutrient magnesium sulphate in concentrations above to/8192 

 (m = molecular weight) is toxic to most seedlings, the degree of toxicity 

 varying with the type of seedling and the conditions under which 

 growth takes place. It will be shown in the following pages that even 

 such a typical poison as boric acid may, under suitable conditions, 

 increase plant growth just as if it were a nutrient. A review of the 



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