82 BIOLOGY OF PLANTS. 



iment ; thus, if a current of moist air is passed over red hot 

 charcoal, carbonic acid and ammonia are readily formed ; 

 hence it is easy to see, that the atmosphere must be constant- 

 ly supplied with variable quantities of this gas. 



What then is its influence in vegetation ? A full considera- 

 tion of this agency will be reserved to a future section. It is 

 sufficient to remark here, that it is supposed, 



1. To yield nitrogen to gluten and to vegetable albumen. It 

 is supposed to enter the vegetable organs either in a pure 

 state, or in the form of some of its salts, and, by various trans- 

 formations, to yield its nitrogen and perhaps its hydrogen, to 

 the formation of vegetable substances. 



2. To stimulate the organs of plants and enable them to 

 obtain a larger quantity of the substances of which they are 

 composed. But as plants have no nerves, such stimulating 

 effects have been doubted by many. The fact, however, that 

 light, heat and electricity, produce effects upon the functions 

 of vegetables, analogous to stimulants, shows that there is no 

 good reason for doubting, that ammonia and other substances, 

 may exert a similar influence. Liebig thinks that no such ef- 

 fect is produced, and accounts for the powerful influence of 

 ammonia, on the principle of its yielding nitrogen, an essential 

 constituent of vegetable organs. But Berzelius is of opinion, 

 that such stimulating effects are produced, and that ammonia 

 .may act in this way. 



3. But the most important action of ammonia is its influ- 

 ence upon the vegetable matter of the soil, and upon the sili- 

 cates. It causes by its presence or catalytic power the decay 

 of woody fibre, and renders insoluble geine, soluble, and ca- 

 pable of entering the roots of plants. It also acts upon the 

 silicates and aids to form nitrates, especially nitre, (nitrate 

 of potassa,) a salt which, as we shall show further along, ex- 

 erts a powerful influence in vegetation. 



IV. Nitric Acid (aquafortis) is formed in the atmosphere, 

 by the discharge of electricity in thunder storms. The quaii- 



