36 PRINCIPLES OF GARDENING. [CH. I. 



of these gases is necessary for germination, and 

 Scheele was the first to demonstrate that it is the 

 oxj'gen. Achard afterwards proved that seeds will 

 not germinate in nitrogen, carbonic acid, or hydrogen 

 gases, unless mixed with oxygen ; and though Carra- 

 dori doubted the correctness of his experiments, his 

 doubt was sho"\Mi to be groundless by the more accu- 

 rate researches of Gough, Cruickshank, Saussure and 

 others^. Senebier carried his experiments still fur- 

 ther, and has determined that although seeds will 

 germinate in an atmosphere not containing at least 

 one-eighth of its bulk of oxygen, yet that the propor- 

 tion most favourable to the process is one-fouith. Ger- 

 mination will proceed in an atmosphere of pure oxy- 

 gen, but not so readily as when it is mixed ^rith 

 other gases. The same phenomena attend the incu- 

 bation of eggs — they will not hatch in the vacuum of 

 an air pump, nor will the process proceed so satis- 

 factorily in any other mixtm'e of gases than atmo- 

 spheric air. 



It is necessary that the oxygen should penetrate 



^ Althougli seeds will not germinate in an atmosphere of nitro- 

 gen, yet they all absorb a small quantity of this gas when ger- 

 minating. It is a constituent of most young roots^ especially of 

 their spongioles, or extreme points. There is reason to believe 

 that by its aid ammonia is fonned during germination, and that 

 this acts as a stimulant and food to the young plant. Seeds con- 

 taining nitrogen germinate more rapidly than seeds of the same 

 genus which do not contain this gas. 



