Conclusions 95 



manganese and boron compounds are able to increase growth if they 

 are present in small quantities, as they act as " carriers " whereby the 

 various functions of the plant are expedited by the increased facility 

 with which the essential nutritive elements are supplied. The 

 manganese in laccase, for instance, is held to be an oxygen carrier, 

 whereby the oxygen is first absorbed and then released for the 

 benefit of the plant, the manganese being regarded as essential for 

 the functioning of the enzyme. But, if these elements are essential, 

 this theory seems to stop short of the truth. If certain functions 

 are dependent for their very occurrence upon the presence of even 

 minute traces of any element, then surely that element is as essen- 

 tially a nutrient element, as vital to the well-being of the plant as 

 is such an element as carbon or nitrogen or phosphorus, even though 

 the latter occurs in far greater quantity. It is necessary that one 

 should free one's mind from the idea that the quantity of an element 

 present in a plant is an index of its value to the plant. Naturally 

 enough, in the early days of plant physiology, the most abundant 

 elements first engaged the attention of investigators, and they were 

 divided into essential and non-essential, ten elements being classed 

 in the former category. More recent work is beginning to show 

 that other elements are constantly present in plants, but in such 

 small quantities that the older and cruder methods of analysis failed 

 to reveal them, so that until lately they have been completely over- 

 looked in work on plant nutrition. Even yet we do not know which 

 of these other elements are essential and which are merely accidental. 

 While we do know that the ten essential elements (C, H, 0, N, S, P, 

 K, Mg, Fe, Ca) are necessary for the well-being of all plants, it is 

 conceivable that these other substances which only occur in very small 

 quantities may be more individual in their action, and that while a 

 trace of a certain element may be absolutely essential to one plant, 

 that same element may be quite indifferent for another species. If 

 one takes a broad outlook, the two theories seem to be in reality 

 only parts of one, the "nutrition" theory carrying matters a little 

 farther than the "catalytic" idea, broadening its scope and extending 

 its application. 



It seems probable that all the experimental work that has been 

 discussed will prove to be simply preliminary to a far greater practical 

 application of the principle of stimulation or increased growth. While 

 the physiologists have been feeling their way towards the conclusions 

 put forth on this subject, the agriculturists have been discovering and 



