The Various Substances Made by Plants 121 



difficulty in drawing all that they need. As a matter of fact, 

 however, the typical plants take no nitrogen at all from the air, 

 even starving to death for want of a little while bathed in this 

 lavish abundance; and the reason they do not is that they cannot. 

 The most prominent characteristic of nitrogen is its chemical 

 inertness, or reluctance to enter into combination with any other 

 substances, — a circumstance, indeed, to which its abundance 

 in the atmosphere is due; and its union with oxygen or other 

 substances can be effected only by the agency of electric sparking 

 machines, or other methods involving the expenditure of high ten- 

 sion energy. Now our typical large plants have not in their struc- 

 ture any equivalent for sparking machines or other arrangements 

 releasing suitable energy, although, as will presently appear, the 

 lowly Bacteria seem better provided in this particular. Since they 

 cannot make use of the free nitrogen of the air, plants have had to 

 resort to the only other possible source of supply, viz., substances 

 in the soil containing it already combined, which substances, 

 moreover, must be soluble in water to admit of their absorption 

 by the roots. The compounds called nitrates best meet these 

 conditions, and they, accordingly, are the source of most of the 

 nitrogen which, with appropriate intermediate chemical steps, 

 is combined with the elements of the carbohydrates to form 

 amides. 



If nitrates were as plenty in soils as plants could make use of, 

 then our digression in pursuit of this substance could end right 

 here. But in fact the nitrates in most soils are so scant that the 

 majority of plants live all the time in touch with nitrogen scarcity, 

 and this is one of the chief of the factors which limit the luxuriance 

 of their growth and expansion. It is, perhaps, worth noting in 

 passing, that especial scarcity of nitrogen in some situations 

 is correlated with an insectivorous habit in plants which reside 

 there, — the advantage of this habit consisting in the abundance 

 of combined nitrogen obtainable by digestion from the bodies of 

 insects. A chief reason for the scarcity of nitrates in the soil lies 



