48 PROPAGATION OF PLAKTS. 



plieric nitrogen, others have proved, to their own satis- 

 faction at least, that they do, under certain conditions, 

 obtain considerable quantities from this source, and 

 Adolf Mayer, in some experiments made a few years ago 

 on air-plants, found that xujrogeny in.ihe form of Ammo- 

 nia, was absorbed in appreciable amount by both leaves 

 and roots, but most freely by the latter. 



Nitrogen forms nearly four-fifths in bulk of the atmos- 

 phere, and is also abundant in all animal tissues, which, 

 during decay, give off nitrogen, combined with hydrogen, 

 in the form of ammonia. The latter is readily absorbed 

 by moist carbon (charcoal), and by carbonaceous matter 

 generally. In this form, plants take up nitrogen quite 

 freely through their roots ; consequently, ammonia is 

 valued highly as one of the most powerful and stimulat- 

 ing of fertilizers. Nitrogen and the oxygen of the air, 

 under certain conditions combine, forming nitric acid, 

 and this, in combination with alkalies, forms nitrate of 

 soda, of potash, and of lime, all of which are useful fer- 

 tilizers for plants. The guano deposits on the islands of 

 the Pacific Ocean, the nitre beds of South America and 

 other countries, are all drawn upon by civilized nations 

 for providing nitrogen and other important elements re- 

 quired by cultivated plants. Not only is nitrogen sup- 

 plied to plants by the application of nitrates to the soil, 

 but in various other forms of animal and vegetable ma- 

 nures, as produced on the farm and in the garden, and, 

 in addition, it is being constantly deposited in the soil 

 wherever animal or vegetable matter is undergoing de- 

 composition. 



These four elements oxygen, hydrogen, carbon, and 

 nitrogen are generally recognized as the four elementary 

 constituents of plants, supplied principally in the form 

 of carbonic acid, water, and ammonia. In such forms 

 or combinations they all exist in. the air as well as in the 

 .earth, hence the means of subsistence of plants that live 



