332 ASSIMILATION. 



contributed in the rain as 1.08 pounds per acre. u We may 

 probably take 4.5 pounds per acre as the best estimate we can 

 at present give of the total combined nitrogen annually supplied 

 in the Rothamsted rainfall. This is only about two thirds as 

 much as the earlier results indicated as due to ammonia and 

 nitric acid alone. ... In addition to the combined nitrogen 

 carried down from the atmosphere in rain, we have to consider 

 an}' gain to the soil or to the crop by direct absorption of am- 

 monia or nitric acid from the air. As far as an} T gain from the 

 atmosphere to the plant itself is concerned, there is very little 

 direct experimental evidence on the point, but such as is avail- 

 able would lead to the conclusion that its amount is practically 

 immaterial. As to the amount of gain by absorption b} T the 

 soil, there is unfortunately no direct or satisfactory evidence 

 at command. From such evidence as does exist, we are 

 disposed to conclude that with some soils the amount will 

 probably be greater and with others less than that supplied by 

 the rainfall." 1 



S73. Direct absorption of ammonia by leaves. Under certain 

 circumstances ammonia can be absorbed directly by leaves. This 

 will be further adverted to under "Appropriation of Organic 

 Matters." 



874. How the nitrogen compounds of the atmosphere are formed. 

 It is a familiar fact that under certain circumstances the free nitro- 

 gen of the atmosphere can be made to unite with oxygen for the 

 production of nitric acid; for instance, by the passage of a spark 

 of electricity through a confined atmosphere a small amount of 

 combined nitric acid ma}' be formed. The bearing of this fact 

 upon the existence of nitrogen compounds in the atmosphere is 

 very obvious. Schlcesing, 2 in an interesting study of the nitro- 

 gen compounds of the air and soil, attributes to the atmosphere 

 a veiy important office in forming and distributing nitrogen com- 

 pounds. According to him, the nitric acid contained in rain- 

 waters on escaping from the soil, where it is only lightly held, 

 finds its way to the sea, where under various agencies (notably 

 that of vegetable organisms of the lowest grade) it becomes, 

 sooner or later, changed into ammonia. This readily escapes 

 into the air, and is carried in the atmospheric currents to all 

 parts of the world, becoming thereby available to land plants. 



1 Journal Royal Agricultural Society, vol. xix., part 2, 1883. 



2 Comptes Rendus, tome Ixxxi., 1875. The same idea has been more or less 

 treated by others. 



