130 FOOD OF PLANTS. 



from the age of the seed before exposure to the agents that 

 produce germination, and the influences to which it has been 

 exposed, whether its vitality may have been injured by mois- 

 ture, heat or light, all of which exercise a deleterious influence 

 on seeds. 



168. The time tbrougli which seeds will retain their vitality, is 

 very different in different species, when exposed to the same 

 influence. We believe, however, more depends on the action 

 of elements of the atmosphere and light, than upon the neces- 

 sarily limited time of suspension of vitality in the seed itself. 

 We have known seeds, which are generally considered as 

 losing their vitality at the end ofone year, readily germi- 

 nate after being kept for several years in a dry atmosphere, 

 and of nearly uniform temperature, and protected from the 

 light. Seeds which are generally considered as retaining their 

 vitality only one year, have been known to germinate at the 

 end of owe hundred years, and cases are often recorded of 

 seeds germinating after fying buried in the earth beyond the 

 reach of the atmospliere, for at least seventeen hundred years, 

 Anv table on this subject, theretore, is necessarily a very im- 

 perfect guide, unless it should be based on some specified 

 manner of keeping the seeds. The best course to follow in 

 the preservation of seeds, is to keep them as much as possible 

 in a temperate and dry atmosphere, protected from the action 

 of the light. In the purchase of seeds, kept in the usual man- 

 ner, fresh ones should always be required. 



Section 6. Food of Irlants. 



169. The principal food of plants is water and carbonic 

 acid, which are received through the roots in a liquid state, 

 and through (he leaves in a gaseous form. Besides these nitro- 

 gen and various salts enter in a greater or less degree into 

 the composition of vegetables. 



Carbonic acid seems to be the most essential article 

 of nourishment for veiretables : since thev will not thrive in 

 a soil or medium in which this gas is not founder generated, 

 and just in proportion as this gas is iurnished to the roots, in 

 the same proportion does the plant become vigorous and thri- 

 ving ; hencti articles, which produce or undergo fermentation, 

 are the most powerful manures, hence yeast is said to be the 

 most efficient manure that can be applied. This results from 

 its power of creating fermentation in the organic substances 

 found in the soil, and thereby yielding carbonic acid. Car* 

 bon in its uncombined state, seems unfitted for nourishing the 



