THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 135 



It is to BoussiNGAULT that we owe our knowledge of the importance of 

 nitric acid as a nutrient to the green plant ; previous to his time it was believed, 

 mainly owing to the influence of Liebig (1840), that ammonia was the chief 

 source of nitrogen to the plant. This conclusion was readily arrived at because 

 experience had shown that excellent results could be obtained by manuring 

 with ammonia ; it was not known that the ammonia in the soil is transformed 

 into nitrate before it is absorbed by the plant. This nitrification (Lecture 

 XVIII), due to the action of organisms in the soil, complicates the scientific 

 explanation of the question as to the significance of ammonia in the nutrition 

 of the green plant. The recent comprehensive researches of Pitsch (1887- 

 1896) and of Maz^ (1900) have conclusively proved that the nutritive value 

 of ammonia must not be entirely denied ; in the majority of green plants it is 

 second only to nitric acid in value, inducing a definite development and con- 

 siderable increase in dry weight. The fact that many plants thrive only 

 moderately well when supplied with ammonia is accounted for by the fact 

 that the ammonia salts when presented to the root in a more concentrated state 

 produce an injurious effect. Carbonate of ammonia, on account of its alkaline 

 reaction, is especially liable to cause injury to the plant, acting as a matter of 

 fact like a poison. In the case of some plants, particularly maize and other 

 Gramineae, ammonia is by no means of inferior value to nitric acid, for Maze was 

 able to obtain as great an increase in dry weight in maize, using at most a ^ per 

 cent, solution of ammonium sulphate, as when he supplied it with a solution of 

 a nitrate. Similar results were obtained in cultures of Brassica and species of 

 Allium. Forest trees also must be dependent on ammonia, since nitrates are 

 seldom present in woodland soils. The significance of this is not so simple as 

 it appears and will necessitate inquiry later on (Lecture XIX). So far as we 

 know at present it is quite certain that in addition to plants which definitely 

 prefer nitric acid (e. g. buckwheat, potatoes, turnips) there are others which 

 get on just as well or even better with ammonia, so that it would appear to be 

 a matter of indifference to such plants whether the ammonia is supplied to 

 them in the form of a sulphate, a nitrate, or a phosphate ; it is only the 

 carbonate which, as already mentioned, is liable to produce injurious effects. 



Let us now inquire into the sources of nitrates and ammonia in nature. 

 Minerals which are of purely inorganic origin, and which at the same time 

 contain nitrogen, occur only rarely in nature. Erdmann (1896, Ber. Chem. 

 Gesell. 29, 1710) obtained only very minute quantities (0-028 per cent, or less) 

 of combined nitrogen in perfectly pure primitive rock. Nitrate of soda would 

 at first sight appear to be an exception, but there is little doubt but that this 

 form of nitrate occurs in nature as a product of organic activity [Muntz, 1889]. 



All evidence points to the fact that the total amount of fixed nitrogen 

 available for plant nourishment nowadays arises from the chemical combination 

 of free nitrogen gas. Processes are constantly taking place which result in the 

 combination of gaseous nitrogen, but the converse process also occurs where free 

 nitrogen arises from the decomposition of compounds. Every combination of 

 nitrogen which is effected means a gain of nutrient to the typical green plant 

 from a substance of no nutritive value, and every formation of nitrogen gas by 

 decomposition of a nitrogenous compound means a loss to it. Hence these two 

 processes as they occur in nature, and which we may briefly term nitrogenous 

 gain and nitrogenous loss, are of special interest in relation to the question 

 before us, and demand closer study on our part, though at the present moment 

 we need not do more than briefly indicate the more important points in regard 

 to the problem, reserving further details for study later on. 



Nitrogenous gain takes place under various conditions. Apart altogether 

 from conditions which may be created in a laboratory, there are only two 

 methods of bringing about nitrogenous combination ; one of these, in which 

 organisms play a prominent part, we will discuss later (Lecture XIX) ; the 



