70 ABSORPTION OF AMMONIA BY PLANTS. 



The quantity of rain that falls at York from the first of March to the 

 middle of June — durin/]; which time the grass grows and generally ri- 

 pens — is about five inches.* On a square foot, therefore, there fall 720 

 cubic inches of water, containing 2 per cent, of their bulk, or 14 cubic 

 inches of nitrogen, weighing 4| grains. This gives 28 lbs. for the quan- 

 tity of nitrogen thus brought to the soil over an entire acre. But if we 

 consider how the rain falls in our climate, we cannot suppose the grass 

 in a field to absorb by its roots, and afterwards perspire by its leaves, 

 more than one-third of the whole. This quantity would carry with it 9 

 lbs. of nitrogen into the circulation of the plants — or little more than a 

 seventh part of the 60 lbs. which, as we have seen, are taken off the 

 field in a crop of hay. 



Such a calculation as this affords at the best but a very rude approxi- 

 mation to tlie truth — it seems, however, to justify us in concluding that 

 jjlants can derive from the air, and in an uncombined state, only a small 

 portion of the nitrogen they are found to contain — and that they proba- 

 bly draw a larger supply from certain compounds of this elementary sub- 

 stance with hydrogen and oxygen — which are known to come within 

 the reach of their roots and leaves. 



The most important of these compounds, and those perhaps the most 

 extensively concerned in influencing vegetation, are ammonia and nitric 

 acid, the properties of which have been described in the preceding 

 lectuie.f 



§ 7. Absorption of ammonia hy plants. 



That ammonia enters directly into the circulation of plants is ren- 

 dered probable by a variety of considerations. 



1°. Thus it is found to be actually present in the juices of many 

 plants. In that of the beet-root, and in those of the birch and maple 

 trees, it is associated with cane sugar (Liebig.) Jn the leaves of the 

 tobacco plant, and of scurvy grass, in elder flowers, and in many fungi, 

 it is in combination with acid substances, and may be delected by 

 mixing their juices with quick-lime. — [Schiibler Agricultur Chemie^ 

 II., p. 56.] 



2°. Some plants actually perspire ammonia. Among these is the 

 Chenopodium Olidum (stinking goosefoot), which is described by Sir 

 William Hooker as "giving out a most detestable odour, compared to 

 putrid salt fish." In the odoriferous matter given off ammonia is con- 

 tained, and may be detected by putting a glass »shade over the plant, 

 and after a time introducing a feather moistened with viaegar or dilute 

 muriatic acid. — [Chevalier Jour, de Pharm. X., p. 100.] It is also pre- 

 sent in the odoriferous exhalations of many sweet-smelling plants and 

 flowers. — [Schiibler, I., p. 152.] 



3°. Nearly all vegetable substances, when distilled with water, yield 

 an appreciable quantity of ammonia. Thus the leaves of hyssop, and 



• The result of experiments made in 1S34 by Prof. Phillips and Mr. Edward Gray. The 

 mean annual fall of rain at York is about 22 inches. — (See fiilh Report of the British Associa- 

 tion, p. 173.) 



t It will be recollected that ammonia consists of one equivalent of nitrogen (N) united to 

 three of hydrogen (Hs), being represented by NH3; and that nitric acid consists of one of ni- 

 trogen (N) and five of oxygen (O5), its formula being NO5.— See Lecture III., p. 34. 



