36 PLANTS DO NOT OBTAIN NITROGEN FROM THE AIR. 



been said that the atmosphere consists of oxygen and 

 nitrogen, we might naturally conceive that the leaves 

 absorb this gas, as well as carbonic acid. Experiments 

 have shown that this is not the case to any extent. 

 After many careful trials, it has not yet been certainly 

 proved that any nitrogen at all is obtained by the 

 greater number of plants in this way. If there is, the 

 quantity must be in most cases very trifling indeed. 



b. This is one of the most remarkable points con 

 nected with the nutrition of plants. Here we have, 

 in the air which surrounds the plant, and presses 

 against every part of it, an immense quantity of the 

 gas nitrogen. It constitutes four-fifths of the whole 

 atmosphere, and yet we cannot find that plants absorb 

 it in any quantity whatever. On the contrary, as we 

 have seen, they select out another kind of gas, car 

 bonic acid, although it is present in so small a pro 

 portion as 2 5*3 0th. This shows conclusively that the 

 leaves do not draw in through their pores every thing 

 that is presented to them indiscriminately, but that 

 they have a power of choosing those kinds of food 

 best adapted to their wants. 



c. Thus the smallest plant has the power of doing 

 what man by his unaided senses never has been able 

 to accomplish, and which he has only learned to do 

 by artificial means within a few years. Every little 

 worthless weed by the wayside has its leaves spread, 

 its thousands of mouths open, selecting and drawing 

 in from the passing air food best adapted to its wants. 



As plants obtain, according to the above statements, 

 little if any of their nitrogen from the air directly 

 through their leaves, they must obviously get it in 

 some way through their roots. There are two bodies 

 which are now considered the chief sources of supply: 

 these are called ammonia and nitric acid. 



Ammonia is .a gas, composed of nitrogen and hy- 

 . drogen. We do not find it largely in this shape, 



