GERMINA TION 



137 



interchange of snbstance, due to vital action, between the seed and the 

 surrounding atmosphere, although extremely slight, so long as the former 

 possesses its vitality. 



The evidence as to length of time during which seeds can retain their 

 vitality is extremely contradictory, and the greatest diversity of opinion 

 exists concerning this point. Our best authorities believe that we have 

 no conclusi^■e evidence that the period is longer than about fifty years, 

 although, upon the other hand, Nve ha\-e no positi\'e evidence that it is 

 not ver^■ much lona;er. 



Figures illustmtinj: conunon forms of goriniuation. 



Germination. — Conditions of (.\)»(/««/(o/i.— Germination depends 

 upon (1) a specific temperature, varying for seeds of ditt'erent species 

 and for those of the same species when they have become habituated 



