THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 137 



that a complete explanation of their mode of operation is not as yet possible. 

 We may note only the fact that, despite the numerous researches which have 

 been made during recent years as to the phenomenon of nitrogen fixation, we 

 have no conception of the quantitative aspect of the question, so that it is quite 

 impossible to say whether the fixation of nitrogen gas or its evolution is the 

 more dominant feature in nature, or whether the one process is the exact 

 balance of the other. When one remembers that originally no combined nitro- 

 gen existed on the earth, one is inclined to hold that the amount of combined 

 nitrogen to-day is on the increase, and that as a consequence actually more 

 organisms are able to exist now than thousands of years ago. Without doubt, the 

 amount of living substance in nature depends on the amount of nitrogen, since 

 nitrogen occurs only to a minimum extent in uncultivated soil. 



Owing to the mode of occurrence of combined nitrogen the green plant 

 can take it up in three different ways : 



1. It can absorb it in the form of ammonia or nitric acid from the soil by 

 means of the root. 



2. It can take up ammonia in the gaseous form from the air by means of 

 the leaves. 



3. It can absorb rain-water and nitrogenous substances dissolved in it also 

 by the leaves from the air. 



The first of these possibilities is really the only one we need consider. The 

 power of leaves to absorb gaseous ammonia is undoubted (ScHLo'ssiNG, 1874) ; 

 but the fact that this gas occurs in the air only in quite limited traces renders this 

 capacity on the part of the leaves of no practical importance. On the other 

 hand, large masses of manure may certainly appreciably add to the quantity 

 of ammonia in the air, and it is quite possible that under these conditions it 

 may exert an important influence on the development of many plants 

 (KEENER, 1887). Any such favourable influence, it must be remembered, is 

 only limited, since in higher states of concentration ammonia is very rapidly 

 injurious. The absorption of combined nitrogen dissolved in rain through the 

 leaves is undoubted, yet this amount also is so small that the ordinary land 

 plant may be considered as entirely dependent on that absorbed from the soil. 

 There is a large amount of literature dealing with the presence of ammonia in 

 uncultivated soils, but into the discussion of these researches it is impossible 

 to enter here. It will be sufficient if we refer to some of the results obtained by 

 A. BAUMANN (1887) : 



One kg. of dry mg. of nitrogen as ammonia. 



Loam (derived from granite) . . (Fir Mts. Bavaria) . . . 22-27 



Weathered gneiss .... ... 11-05 



porphyry .... (Rhine Palatinate) . . . 17-71 



,, carboniferous sandstone ........ 4-43 



,, basalt .... (Rhine Palatinate) . . . 23-37 



Loess without humus .... (Munich) ..... 6-58 



Sandy soil (Schrobenhausen) . . . 2-23 



Moorland soil (MQnich) ..... 1-60 



Soils which are unworked and unmanured vary greatly in the amount of 

 ammonia which they contain ; basalt and loam soils contain the most, sandy 

 and moorland soils the least. Further, the amount of ammonia decreases rapidly 

 as the deeper layers of the soil are reached. 



On investigating the amount of nitric acid in uncultivated soils the same 

 author found it occurring for the most part in such minute traces that it 

 was impossible to estimate it quantitatively. On the whole, then, the plant 

 can obtain under natural conditions only a very small quantity of combined 

 nitrogen in the soil, and its growth is thus dependent on the characteristics 

 of the root-system already referred to, more especially the capacity it has 



