46 PRINCIPLES OF GARDENING. [CH. I. 



plant. The quantity of oxygen absorbed by seeds 

 differs in every species, but they entirely agree in 

 emitting it all again in the form of carbonic acid ; it 

 is absorbed therefore for the purpose of diminishing 

 the seed's carbon. 



The seeds of beans and lettuce absorb one-hun- 

 dredth part of their weight of oxygen to enable them 

 to germinate ; purslain, onion, and radish seed one- 

 thousandth only, and the weight absorbed is always 

 proportionate to the weight of the cotyledon. 



The fact of carbonic acid being extricated aids to 

 explain why germination proceeds more slowly in 

 clay soils, and in soils rolled firm, even mider other- 

 wise favom'able contingencies, than it does in porous, 

 well pulverized soils ; not only does the atmospheric 

 air get to the seed in the former soils vdih more 

 difficulty, but in these the carbonic acid emitted 

 during germination is confined in immediate contact 

 with the seed, and M. Saussure found that carbonic 

 acid, almost in any proportion, retards the com- 

 mencement of germmation. 



That the atmospheric air is that mixture of oxygen 

 and nitrogen gases which is most favourable to the 

 due progress of germination, is proved by the experi- 

 ments of M. Saussure, for he found that seeds ger- 

 minating in it always absorbed a portion of the 

 nitrogen, but which they did not do if the proportion 

 of oxygen was increased. 



