122 THE ART OF CULTURE. 



made by the escape of the gas may be distinctly 

 heard at the distance of several feet. Here the 

 carbonic acid rising to the surface displaces com- 

 pletely all the air, and consequently all the oxygen, 

 from the soil ; and without oxygen, neither seeds 

 nor roots can be developed ; a plant will not vege- 

 tate in pure nitrogen or carbonic acid gas. 



Humus supplies young plants with nourishment 

 by the roots, until their leaves are matured suffi- 

 ciently to act as exterior organs of nutrition ; its 

 quantity heightens the fertility of a soil by yielding 

 more nourishment in this first period of growth, 

 and consequently by increasing the number of 

 organs of atmospheric nutrition. Those plants, 

 which receive their first food from the substance 

 of their seeds, such as bulbous plants, could com- 

 pletely dispense with humus ; its presence is use- 

 ful only in so far as it increases and accelerates 

 their development, but it is not necessary, indeed, 

 an excess of it at the commencement of their 

 growth is, in a certain measure, injurious. 



The amount of food which young plants can 

 take from the atmosphere in the form of carbonic 

 acid and ammonia is limited ; they cannot assimi- 

 late more than the air contains. Now, if the 

 quantity of their stems, leaves, and branches 

 has been increased by the excess of food yielded 

 by the soil at the commencement of their deve- 

 lopment, they will require for the completion of 

 their growth, and for the formation of their bios- 



