GERMINATION. 9 



in germination that the seed procures the requisite 

 supply of oxygen ; fixing hydrogen, the other ele- 

 ment of water, in its tissue : and thus it is enabled to 

 form carbonic acid, which it parts with by its respira- 

 tory organs, until the proportion of fixed carbon is 

 lowered to the amount suited to its growth into a 

 plant. 



15. But the formation and respiration of carbonic 

 acid takes place most freely, though not exclusively, 

 in darkness ; if exposed to light, the seed again parts 

 with some of its oxygen, and again fixes its carbon 

 by the decomposition of its carbonic acid. 



16. In addition to this, the absorption of water 

 causes all the parts to soften and expand ; many of 

 the dry, but soluble, parts to become fluid ; sap, or 

 vegetable blood, to be formed ; and a sort of circula- 

 tion to be established, by means of which a commu- 

 nication is maintained between the more remote parts 

 of the embryo. 



17. Heat seems to set the vital principle in action, 

 to expand the air contained in the numerous micro- 

 scopic cavities of the seed, and to produce a distension 

 of all the organic parts, which thus have their irrita- 

 bility excited, never again to be destroyed except with 

 death. What degree of heat seeds find most condu- 

 cive to their germination, probably varies in different 

 species. Chickweed (Alsine media) and Groundsel 

 (Senecia vulgaris) will germinate at a temperature 

 but little above 32° Fahr. 



18. Germination being established by the absorp- 

 tion and decomposition of water, and by the requisite 



1* 



